Let him sleep.
My son was born a bit on the small side, with low blood sugar and a small problem with maintaining body temperature (in the first days). We were told we have to wake and feed him every three hours, mainly because of the blood sugar issue. But after two weeks, we were told this is no longer necessary, and that we can now wait for 4-5 hours between feeds during the night. If your baby is of normal weight and has no health issues, I would guess that letting him sleep as long as he likes is really OK.