N.E.
My son is in a similar situation, in that he is on the skinny side in spite of the fact that he eats very well and is allergic to dairy. One really good food to give is advocado. My son has liked them since he was introduced to solid food, but now I mix it with mashed black beans and spread it on a tortilla or let him dip tortilla chips in it. Sometimes I mix in some shredded vegan cheese as well.
Also, he drinks hemp milk (available at whole foods) as his milk replacement. It's somewhat expensive, but has much more fat content than rice or soy milk (almost as much as whole milk), and all the same vitamin and mineral supplements. I just learned that you get a discount when you buy 12 or more at a time (case discount).