Hi there - you mentioned that he is also being treated for reflux. What with? It's possible that it could be affecting him - not wanting to drink as much...
I'd just be very cautious about overdoing the solids compared to the formula. There's really no way (and particularly with all his dietary constraints) for him to get the nutrients and particularly things like the fats that he needs from the solids you can feed him. I'm rather surprised that your health care providers even suggested the solids at such a young age.