Make the most of it, when you reach middle age you will be asking how to take it off. It sounds like he needs to increase his muscle mass not a tummy paunch.
stir skimmed milk power into everything like puddings and cerials etc. - thats good for putting weight on.
Eat lots of nuts
Generally try and mix sugars/carbs/fats/ proteins in one meal so for example eat lots of white buttered bread with cheap fatty bacon/sausages and drink sugary drinks.
If you want to put muscle on eat mainly lean protein like chicken along with very little amounts of carbs also take supplements you can get at holland and barratt. And lift weights.