How do i stop my son sucking his thumb? and will it ruin his teeth?

You don't say how old your son is, so it could be his baby teeth or his permanent teeth? Either way, you do feel his teeth are moving forward and as his Mum, you are the best person to know that, so it is probably a good time to stop.
Thumb sucking can push the teeth forward and also contribute to an open mouth posture. The upper teeth develop best with the constant pressure of closed lips and breathing through the nose. Establishing this pattern will help your son enormously in all aspects of his oral development. 
Explaining your concern and getting your son on-side to kick this habit is way more powerful than imposing a new rule! The stimulation his brain receives from the sucking needs to be replaced. Keeping his hands busy can often do this so let him choose a new sensory toy or two, which give him lots of sensory input which doesn't involve the mouth.
Wrap the thumb in micropore tape and draw a little smiley face in water soluble texta, which gets redrawn after hand washing. Every morning the face is intact he gets a sticker to add to a reward chart, with rewards at say, 3 days, 7 days, 14 days etc. Keep remnding him what HE is working towards and encourage him to ask fr your help if he is struggling. 
He may need you to sit with him while he goes to sleep at first and give him lots of extra hugs and physical interactions. Encourage him to use his mouth more appropriately with adventurous textures in food, play 'taste and smell guessing games, tongue twisters, sing songs with animal noises, and vehicle sounds. Substitute beat boxing if more age appropriate!
It may take time and effort but it is well worth it to promote balanced lip muscles and good mouth posture.

PLease feel free to contact and ask further questions 

Kerry has made some great suggestions for managing your son's thumb sucking. It is also useful to look at orofacial muscle habits in detail that may have developed in conjunction with the thumbsucking and open lip posture: to see if retraining for the tongue lips and face is required in conjunction with cessation of the oral habit. Established muscle habits do not usually correct themselves even if the thumb sucking is habit is extinguished and can continue to impact dental and facial development when left uncorrected. Resting tongue posture and tongue movement during chewing swallowing and speech are all potentially affected by a thumb sucking habit. A Speech Pathologist/Orofacial Myologist skilled in this area will be able to help you with both assessment and treatment.

From psychological point of view the thumb sucking could be a control pattern your son developed to repress difficult emotions. Everyone experiences difficult emotions and releasing them through crying is very beneficial for babies. Does he suck his thumb more often when he is upset? Try to give him your full attention next time when he does it to check if he needs to release some bottled up emotions. Try not to pull his thumb away from his mouth just give him your full attention, hold him in your arms and ask if he is upset and assure him that it is ok to cry.

I would like to comment on the dental and orthodontic aspects of thumb sucking;

Your child is proberly sucking their thumb because it soothes and comforts them, yes it is a problem after the age of 2-3 but we have to be realistic, you are generally going to cause the child more traums by trying to get them to stop than by letting them grow out of the habit naturally.

Unfortunately they will need more orthodonitc treatment and it will cause the upeer arch to narrow and possibly an anterior open bite etc, but generally they are all correctable early and easily you just have to be aware of it.

In terms of the germs on the thumb have a close look it will be the cleanest finger on their hands and if the dirt they licked off the floor or the food they found under the couch that must have been there for a week didn't hurt them I am sure the germs on their beautifully clean polished thumb isn't going to hurt them.

Having three children of my own I always advise my parents to pick their battles, try and get them to stop but in the end of the day we can always fix the problems.

I come from the perspective of seeing many kids, teens and adults in my orthodontic practice who fit into 3 categories: are still thumbsucking, was thumbsucking and stopped and thumbsucking continuing into late teens and early adulthood.
Does thumbsucking matter?
After the age of 4, thumbsucking or fingersucking will start to have an effect on your child's bite ie: an overbite of the front teeth can or will develop and a narrow upper jaw can result. This can result in a lisping during speech which my local speech therapist we work with states is best addressed when young,
What ways we have tried that have stopped thumbsucking?

• Gradual weaning down. Most toddlers do it when sleeping or relaxed and not during preschool/school. praise  them when you continue it out of the mouth
• Positive reinforcement with small tangible rewards words better than negative discouragement. remember to recognize this positive behaviour with small treat or reward.
• If this 2 fails- try placing a bandaid over the sucking finger/thumb . Sometimes using a bitter nail coat (Please consult with your pharmacist on how to use first and when- Stop N Grow is one example)
• If this fails, try a thumbguard ie: https://tguard.com/. This is a removable device that covers the thumb. We have a successful result in 1 child where all the previous above failed.
• If all this fails, then please see an orthodontist to have a orthodontic appliance fitted. This appliance does not stop thumbsucking but discourages this habit by making it uncomfortable to do so. Even where we made one, we still have to employ the above strategies or have parents assist us on the above to make it a success.

Thumbsucking after the ages of 5 and above will start to have an effect on the bite. The earlier it can be stopped the better. If the overbite and crossbite effects on the teeth do develop, they are best treated early before late teens or early adulthood. This would keep orthodontic treatment options simpler and more successful, which means to parents less cost, time and stress and ultimately a happier patient.