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 150-200g of vegetable porridge. He's a big eater and never seems to have enough. He loves his veggie porridge and quickly finishes 200g of it. After his 180 ml bottles, sometimes he's content, other times not at all. So after his veggie porridge, he doesn't ask for anything more.
He has severe reflux, but so far the veggie porridge has stayed down perfectly, which is great news.
However, since starting the veggie porridge, he has very hard stools. I'm now replacing one bottle with fruit porridge 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 struggling with this dilemma:
- Are his 180 ml bottles too small? Should they be 210 ml? But he's already so heavy... and he keeps eating
- Is he getting enough fluids from his veggie/fruit porridge? Although he absolutely doesn't ask for milk after these meals, I read that some people give an extra 120 ml of milk on top...which seems like a lot to me if he's already consumed 200g.
- He gets a little water after eating, but it's really just about 20 cc or so

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

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