Difficulty gaining more weight ?

my friend asked me why he isnt gaining much weight...i mean he eats more than me. he really could eat more than every one at our lunch tabe and he works his *** off cuase hes my fittness partner and has been for 3 years now. he says he wants to do football next year, and be a lineman. and im not sure what to tell him i mean what whould be a good supplement to use or to gain weight other than eating protien