Try not to stress
Didn't want to leave you unanswered.
My dd (2nd dc) was similar, though without the allergies. She was 8.5lb at birth so 75+ centile.
She was nearly 7 months before she could take more than 3-4oz. She fed every 4 hrs in the day, would wake at least once, mostly 2-3 times, a night. Wouldn't dreamfeed.
She didn't take to solids either really and only in the last few months (she's 18months now) has she finally got an appetite.
I worried a little, as my first was an eating machine, but I just had her weighed regularly, maybe monthly, and let her be.
If she'd been my pfb though I might've been much as you are.
Not overly helpful but, if he's not losing weight, maybe he's just not a big eater. They really are all different and haven't read the side of the formula box to see what they should be having.
Is there an alternative formula though? Maybe he just doesn't like the taste of it.