Having a "temperamental" veiled is often just luck of the draw. Chams are intelligent enough to be individuals, regardless of genetics or environment. What you can do is recognize what seems to irritate him more often and adjust your activity around his territory accordingly. How old is he now? When their hormones start kicking in they can go through a pissy teenager phase but mellow out slightly as they age. Very hard to predict. You won't really force him into being nicer but you can teach him that you are not a threat, that you are a source for good things by hand feeding him favorite treats and respecting his space. You may not be able to handle him as often as some other people do. If he is pissy right when you approach him or try to pick him up maybe you are handling him too often.