There is a natural one that works great but if your chameleon is really infected I would take him to the vet. It's called worm guard and it has black walnut and cloves which both kill parasites. Good luck!
So if you get "a" dewormer, you will eliminate some species of parasite but leave any others that are not killed by that dewormer there??
IMHO it makes more sense to find out what (if any) parasites are there and kill them.