Sorry, it's been hard for me to get back here, but I wanted to thank everyone for their ideas and thoughts here.
A few things...no I am not breastfeeding any longer. It's a very emotional topic for me, but basically I planned to bf untilat least 2, older hopefully,and I had to abruptly quit at 6 months. It was traumatic (for both of us) and awful and I feel guilty every day about, but there was no choice,I was very sick.
Anyway, he had been sick off and on for a few weeks when I was very worried about this, but he had gained back the 1-2 lbs he lost by his 15 month appointment last week, so I feel better and I don't think he's "not thriving" on this diet.
He eats a McDougall diet basically, but I add more fat (vegan butter, oils, snacks with higher fat content, etc). And, while he's not breastfed anymore, he is consuming about 16-20 oz of hemp milk each day which has essential fatty acids, B12, etc. I'm not going to give him meat or eggs just to add a few lbs to him when there are other ways to consume fat. I just don't believe any animal products are beneficial or okay to eat. Oh, I also started giving him coconut milk yogurt - he loves it and it's pretty high in fat.
I can't remember if I added this to the OP, but he was only 4 1/2 lbs when he was born (full term). So he gained A LOT of weightin the first 6 months to "catch up". He was at negative % on the charts at birth and got up to about 6-10% and is now in the 1-2%. So while he has dropped, I still don't feel like weighing 19 1/2 lbs at 15 months old is THAT bad for a 4 1/2 lb baby. He's not destined to be a linebacker