You will need to speak to the Dr about the dose, but on the "crying over spilt milk" site (it's a NZ site) it gives a low dose of omeprazole (which is what prilosec is) at 5-11.25 mg for 7.5kg weight. I know he's on the 5mg dose twice a day which puts him at the higher end of the low dose range, but if each individual dose isn't high enough he won't be getting any benefit from it. A high dose for the same weight is listed as 11.25 - 22.5mg.
As for the milk intake it is very common for refluxers to take in less but it's only a worry if they aren't gaining weight or meeting milestones. My two were both big babies and took an average of 20-25oz a day - DD in particular stuck like glue to the 91st centile on that intake. Solids can actually help them take more milk in some cases as it helps keep the milk down, they reflux it less so feel more like eating. Solids did make my DS worse, but he wasn't medicated at the time.
How much is he taking in 24 hours, which formula is he on and which flow teat?
Laura