I remember watching this horror movie when I was a kid, it was about a kid that turned in to a monster? I think at the beginning he was sick, and they gave him injections for the disease, but later on in the movie he finds out they were just water.

These other parts I also remember but I’m not 100% sure they’re the same movie: I think he might have been growing too fast, and that was the “disease”, I think at the beginning there was a scene of a birthday party, and I think at one point a lady  and him are trying to escape or run away or something and she goes to get a hotel room and he gets out of the car??

I’m 21 and I saw it as a kid so I’m guessing it’s from the 80’s or early 90’s, it was in English, and in color.

Hope you can help me and thanks in advance!