There was a scene in a 1970 horror or sci-fi movie where a retarded kid or teenager is playing with an air gun and some bad guys decide to throw him up against a car and pour battery acid on his face.
I think he then becomes the henchman to a scientist.
Could it be H(1973) Horror High?
“Brain of Blood” (1971)?
It has a slow man in overalls who plays with an air gun. You can see the acid scene in the trailer below. It starts at 20 seconds in.
https://youtu.be/uCPeRiCXlBc
YES! Brain of Blood! Thank you.
Man that scene freaked me out as a kid.
Thank you Kagan!
Solved!