I have been racking my brain for ages now trying to remember the title of a film my dad and I watched when I was a kid. I think it was early 80's. I think the title was something along the lines of "why did you have to pick on me" but I just can't remember.
I can't remember much about the film other than it was about a bloke who gets lumbered with this young boy. I remember the boy had weird powers or something.
The only thing that I know for sure happened in the film is, the man takes the boy to the doctors to get a jab (can't remember why). The doc tries to throw the needle into the boys bum like a dart but it hits his cheek, bounces off and sticks in the wall.
Please help.
P.S. I'm not sure but it's possible there was a sequel

You pretty much got the title correct. Almost certainly, the movie you remember is an English-dubbed Italian flick from 1980 called Chissà perché... capitano tutte a me (aka Why Did You Pick on Me?). An imdb.com reviewer notes: "Basically the kid is an alien and Bud is a sheriff (this is a sequel to The Sheriff and the Satellite Kid, which I now also have gotten from ebay on the strength of this) who is protecting the kid from the government."