Answer:
I understand your desire to have a positive relationship with your budgie. I owned cockatiels for nine years and I know learning when and how to pet them can be difficult when you're first starting out.
It's great that he trusts you, but it's probably better if you stick with petting his head. Birds are generally uncomfortable having their bodies touched, and in fact if he does let you pet his belly, back or wings, he will likely take it as sexual innuendo (this mostly happens with larger parrots but budies are susceptible too)--your bird will probably end up wanting to mate with you, and can get frustrated by your "mixed signals" when you don't do so! Sexual orientation doesn't seem to mean much to birds--I think they have as much trouble telling us apart as we do them!
As long as you're being gentle I doubt you're hurting him. More than likely he just wants to rest his head on the perch while you get some good scratching in. Since birds have delicate necks this lets him stabilize himself and still enjoy the attention. So don't be afraid, take it as an open invitation. If he wants you to stop, he'll either step away, jerk his head back, or give you a light peck.
Good luck with your keets!
