Anyway to convince an awake baby to go to sleep?

My son is 5 1/2 months and almost always up over midnight, he just doesn't seem tired. I can't let him cry as I live with my parents at the mo and it isn't fair on them and sometimes he's not upset anyway, he just wants to play? I've tried lullabies and all sorts like that. His bedtime routine is bath then bed with a feed if needed, he'll sleep for 3/4 hours and then wake up for 2 hours over midnight.