Now, before I launch into ways to fatten up your man, I think its important to consider that maybe he's happy the way he is, and if he is, then maybe its an idea to accept him like that (unless he's unhealthy kind of skinny, in which case you/him should seek help).
It depends, if he really enjoys food, then ask him what kinds of things he likes and try to make them for him - or if you're a bit of a cooking disaster, take him out to eat or ask his family/friends for tips.
It's important not to say anything directly to him about being skinny, you don't want to make him lose confidence.
If he doesn't enjoy eating, then you could always associate food with sex? get him to eat chocolate off of you and chances are, he'll enjoy it :P
Its also important that while you want him not to be skinny, don't try to get him to be unhealthy, there are lots of ways to fatten up your boy without ruining his health.
Family doctors can be very helpful with personal diets. A lot of people people don't what they should be eating And... a persons body reacts and deals with certain kinds of food so differently the rate at which calories are burnt (metabolism) varies.
It would be cool if there was a set of rules that worked for everyone, but really this is not the situation . So... best bet is to suggest to him that he might benefit from seeing his doctor or a nutritionist.
Oh, one last thing... most importantly some people have deficiencies in their system or may require certain vitamins or minerals, supplements that will help to digest and utilize proteins and fats. Most people won't be able to figure that one out without the help of a medical professional either .