A.N.
I agree with one of the other posts, about letting him pick out a "nice shirt." My son is also 3 1/2 and I always have to give him choices when picking out his clothes or we will fight about it. It seems at this age, they are learning that they have their own voice and want to make their own decisions. Guide his decision if it is not an appropriate shirt, maybe give 2 suggestions and he can pick which one, and let him know that it has to be nice for the wedding, but that he gets to make the decision as long as it's appropriate. This usually works for my son, and if he sees Dad wearing a "nice shirt", it usually helps.