A full grown lionhead is about 3lbs. and seeing that he's about 2-3 months, I would guess he'll be 4-5 lbs. full grown.
But there is the possibility he's older than you were told.
I checked his feet and the heel is still covered with fur and very soft. He has not callus. I think he is pretty young. Once he is settled in we will get more info from our vet.