Why not give him a 75% share on the condition you do not play an active role in running the forum but get 25% of any profits made. Then if he makes money you get a few quid if not then who cares, not like £300 really is that much anyway. Last thing you want to do is give away something with potential for a few hundred quid when it could earn you than that if resold to a forum buyer in a few months or with a bit of successful advertising. 
