I swear by a protein shake/weight gainer called Lean Mass XT by Matrix Nutrition, my gains whilst using it are much better than when I'm not using it. Plenty of calories, protein and carbs to help build muscle. I of course eat a lot of food along with it, but if you genuinely have trouble eating a lot then this may help too. You can get a massive 6.8kg tub from several sellers on eBay, costs about £45 and lasts a month and a half if you have 2 shakes a day.