I wouldn't feed at all if he is coping like that - but that's my personal opinion. Mine was also only on DF at that age (from 12 weeks I guess when I switched 1x NW to 1xDF) and sleeping through till 6 or 7am. 5/6am wakeups are common at that age - and in our case feeding didn't help at all, even make things worse, feeding at 5 meant he wanted to start a day at 5...:/. I guess it is just their age, they start to wake after 10h of sleep and should coo in the crib and if everything is fine, they can go back to sleep after that for another 1-2 sleep cycles.