20oz is I believe the low end of recommended for this age but perhaps that's ok for him.
How do you settle him at night wakings?
My HV told me that 18-20oz was the minimum but that this was still plenty to be having. He is a big boy, especially for saying he was premature, but has never drank more than 5oz in a feed. He's never been a big fan of milk - I put it down to him having undiagnosed milk allergies for 6months and now associates milk with being uncomfortable and just drinks enough to get by!
During NWs he has a dummy which I replug (although when he self settles he doesn't always have it so I know he's not 100% reliant) and sssh pat. On most NWs before 4am this is a very quick process - less than 2mins. After 4am the dummy sometimes doesn't cut it and he continues to moan, the sssh pat takes longer or doesn't work at all. If I am unable to settle him in his cot after 10mins I tend to try PUPD but he's quite heavy for that now and I end up just holding him or co-sleeping once it gets to 5:00am.