Your boy is healthy, right? Then he's probably OK. Food is an up and down thing. Babies eat and drink more as they grow, yes, but that's in the long run. In the short run, they can... well, do things like this. As long as he's healthy and happy, he's obviously getting enough nutrition.
You might talk to your doctor about what to do for him instead of formula, since he doesn't want it. You don't want to start a big food war, because you can't win food wars. Pick your battles carefully! (You'll know this more certainly when he's about two years old.)
You may have read all the right books (as I did) and know what OUGHT to happen, but your baby hasn't read them. So smile! Offer him regular meals and snacks, but when he doesn't want them (or want much) then declare meal time over and let him down. When he is hungry, trust me - he will eat!