Greetings Lori, I don't have your William Forman specifically but I do have a Forman family from Westchester. My ancestor is James Forman (b 1750 in Cortlandt) who went to what is now New Brunswick as a Loyalist (fought for his rightful king in the revolution) in 1783. He had a brother, Jacob Forman (no other details) who could be the father of your William. Having the name might help your search. (might not, too)
I have some information on their pedigree if you think it might help.