How old is he now? 5 months? What's your feeling about solids? I think I would consider starting one meal a day and seeing if that helps, I wouldn't be rushing into 3 full meals but he is taking a lot of formula. I started DD at 17 weeks (under Drs instructions), for reflux but she coped well, she was grabbing food off our plates at 5 months too and getting it to her mouth!
You could try hungry baby milk but all that does is give him milk that is harder to digest and he's going to have to adapt to solids soon anyway so is it worth it IYSWIM?
Laura