Swetha,
I just want to point out that if your son is on a very strict GF, DF, and egg free diet, and is vegan, he might be severely deficient in B12 and other B vitamins. This alone can cause tics!
If he is positive for MTHFR, the problem is even more so, he will need to supplement with a good Methyl B Complex.
I have seen very good results with supplementing with B complex patches from the company Nutripatch. Just be aware, diagnosing this issue is not so easy and is not a simple blood test. (although sometimes it will show up on regular bloodwork)