lacto-ovo vegetarian needs to gain weight...

My boyfriend needs to gain at least 20 lbs. We eat a fairly well balanced diet but he can't seem to gain an ounce. Neither of us is really into tofu or soy...he does drink chocolate soymilk now and then as well as some soy burgers. I am more of a vegetable person...I gave up meat and have no desire to replace it with fake meat (that and it gives me major gut rot). What is there in terms of supplements or food combinations that will flesh him out? He gets lots of protein in the form of nuts and rice/bean combos. Doesn't eat sugar, no coffee and little in the way of junk food.

Any help at all is greatly appreciated.