15 month old, slow gainer...

my baby is 15 months old, I am breastfeeding still. He was 6lbs6oz at birth, and now at just about 16 months he's only 19lbs. He was doing awesome with weight and growth till he hit 8 months. I tried giving him milk based formula, but he started having watery stools. Myself, as well as my other 3 kids all have lactose intolerance. Then shortly after my son got 3 ear infections 1 right after another. He is not gaining weight like he should, and this has put a major delay in his development. At almost 16 months old, he is just walking around furniture, only says dada, no actual words yet. My pedi says he's fine, just small, but I feel there is something not right. I don't think he's absorbing his food, can someone offer SOME sort of advice?