Hi! Both my husband and I are vegan.
I make a lot of bean recipes and often serve them on top of greens or green beans for myself but will make rice or other grains for my husband and bulk up his meals a bit that way.
I also give him snacks to eat during the day including almond butter sandwiches and soy yogurt with frozen blueberries.
He also likes to snack on tortilla chips so I'll often buy tortilla chips and salsa for him to eat. He also snacks on almonds as well.
So basically I'd say make the same basic meals but add more grains and high calorie foods to his meals including things like avocado and nuts. You could even make him trail mixes of mixed nuts (I buy raw) with dried fruit.
(anyway, I was trying to say calorie counting is probably the way to go
)