I agree I would think he would need to maintain a decent amount of cals to maintain preformance in his sport which is always key with an athelete. Simply have him clean up his diet. I would think that with his activity level just a simple change in food choices as well as a solid lifting plan he would make major progress.
Check out Westside for Skinny Bastards by Joe DeFranco, my favorite training program ever published on this site. You might have to scale back some of the extra running and drills that Joe prescribes if he is already doing alot of that stuff for Rugby!