Still giving milk after veggie/fruit puree??

Hi everyone,
My little boy is just 5 months old now. Since he was 4 months, he's had 4 bottles of 180 ml per day plus 150g to 200g of vegetable puree. He's a big eater and never seems to have enough. He loves his veggie puree and quickly finishes 200g of it. After his 180 ml bottles, sometimes he's content, but other times he's absolutely not. So he doesn't ask for anything more after his puree.
He has strong reflux, but so far he keeps the veggie puree down perfectly, which is great news.
However, since starting the puree, he's had very hard stools. I'm now replacing one bottle with fruit puree, and I'm worried that only 3x 180 ml of milk per day won't provide enough fluids and will make the problem worse. He's also not exactly a small guy (8 kg, 68 cm - 5 months). So I'm constantly facing this dilemma:
- Are his 180 ml bottles too small? Should they be 210 ml? But he's already so heavy... and he can keep eating.
- Is he getting enough fluids from his veggie/fruit puree? Although he absolutely doesn't ask for milk after this meal, I read that some people give an extra 120 ml of milk on top...which seems like way too much to me if he's already devouring 200g.
- He gets a little water after eating, but it's really only about 20 cc or so.

Any tips? Should I increase the bottles from 180 to 210, or give 180 ml milk + dilute with 20 ml water, or maybe give extra milk after his puree?

It's the dilemma of whether it's too much or too little, so :-D