My DD did well with avocado (ripe) and whole milk yogurt. When she was around a year old, she got the "failure to thrive" diagnosis due to low weight. Doc recommended Carnation Instant Breakfast. Um, no, thanks. Aside from that one visit and issue, this doc was/is really great! I found it odd.
Anyway, that was 7+ years ago and I have learned a lot since then. I bet the other mamas here will have lots of great ideas for you. Coconut oil comes to mind, but I am too far removed from babies to know what is good for LOs these days. I saw in another thread that your little guy is 14 months and has liver disease. Best wishes in finding what works!
"Kind words can be short and easy to speak, but their echoes are truly endless." - Mother Teresa