We had this. DD was diagnosed with CMPI at ten weeks. Weaning was a nightmare. Turned out she was intolerant to loads of stuff, including gluten and soya, but also stuff like avocado, banana, grape, all orange fruits and veg, and loads and loads of other foods. We are starting to come out of it now at nearly three, and she can now tolerate foods she previously couldn't.
The best thing is to take it slow, only introduce one new food at a time, tiny amount on the first day and then getting bigger and bigger over 4-5 days. If no reaction you can start introducing a new food. If reflux worsens, stop the food and wait for the reflux to subside until you try something else.
Is your DS being seen by an allergy clinic? He really should be, and it will give you access to dietitians too.
We had the most luck with gluten-free oats, rice, potato and green fruits and vegetables. Apples, pears, raspberries, blueberries and prunes were all ok. Cauliflower, broccoli, peas, sweetcorn, green beans and asparagus were fine. All meats and salmon were ok, but sardines were not. The problem with orange fruit and veg seems to be a known thing, though not common. Avocados and bananas do seem to be a problem for some. Annoying!
Pear is known to have a low potential for intolerance. Cauliflower is very gentle. Lamb is meant to be the meat that is most widely tolerated. I would try them next.