We had exactly the same with ds1, and the paed (a specialist in reflux) told us to do this:
A few spoonfuls of baby rice before every milk feed.
Introduce a new food every three days, to see if he reacts negatively to one of them.
Savoury at lunch, fruit in the evening.
Fruit: home made pureed apple, pear, banana to start with.
Savoury: home made potato puree, then add 50% of carrot (very well boiled), then iirc squash fennel erm....sorry, I can't remember (but you get the general idea, very cooked bland vegetables mixed with potato puree).
Immediately after that, start mixing it with some meat. Chicken breast, cooked pork or lamb (not beef of course).
Ds1 also did really well with avocados.
FWIW, ds1's weight picked up dramatically once he was on solids and on the drugs. He went from being off the bottom of the charts at 4 months to being on the 90th percentile by the time he was a year old! He also came off the drugs completely by the time he was 6mo.
It's hell being where you are and it was the hardest thing I ever went through, but it can and does get better from here.