Looking for some advice.

Bit of background

My lad is 15 and is a little on the small size. He plays rugby both league and union and is a good player. st helens are watching him but have said he isnt ready to step up to the scholarship, they have spoken to me and said he needs to be a bit bigger.
He as next to no body fat and is as fit as you possibly get. he is strong for his size but he gets mistaken for a 13 yo all the time. he is starting to struggle now as the lads are getting bigger.
He does need to build some mass and thats what im asking for help to see if anyone can give some advice.

He does rugby training for his 2 clubs which is 3 times a week at the moment. trains for his school once a week and plays on average 2 matches a week. With his fitness and stamina not in question, I need to help him with getting to his goals.

He as a gym membership and is allowed of the resistance machines and not on free weights.

Can anyone help with a training plan and diet?