Talk to a nutritionist - but you might try some of this too.

My hubby has an incredible metabolism - nothing else wrong with him; if you can call that "wrong".

When he used to train, ride all the time and race, he was "incredibly skinny". Now, he doesn't ride, race, etc. that much and is just "skinny".

What we found helped with him, was he would eat regular meals, many meals, not all low fat. He would eat some things that have a good amount of fat - (you still have to be careful with this for the obvious reasons) but don't try to cut out all fat. Make it good fat, but make sure you get enough. And, of course plenty of carbs, protein and fiber.

Also, add supplements - try the after exercise sports drinks. Drink a whole bottle every time you come back from a work out. (these tend to put weight on or at least with my hubby hold the weight). Now, if he feels like he hasn't gotten enough calories with his meal, he will drink one of those meal replacement drinks i.e ensure to help him hold weight.