Should be possible to look at Corey Haim’s movie work around age 10-12 and see who the candidates are, who was involved with the movies of his career at that time.... unless it were someone not officially credited on his films, but that seems less likely???
Well I guess it’s not so easy, I looked at Corey Haim’s first films and did not see too much to go on, if it’s supposed to be a world famous Hollywood name now..... probably have to know the “social networks” around those early days to know who might have had access to him on and off set, etc.
He was born in December 1971 and didn’t start making movies until 1984 (”Firstborn”).
If it was a co-star, the answer is at imdb.com