Can boys wear girls varsity jacket?

It depends on the culture at your school. If there is a distinct jacket for varsity players as distinct from junior varsity, and the freshman is not a part of the varsity team, then probably not.

At my high school, there was just one "letter jacket", which was the same throughout high school, and freshmen who had earned letters could wear it. (I suppose technically you could wear it without the letter, but you'd have looked kind of like a doofus.)