D.W.
He's only 5 months old. He's not supposed to sleep through the night yet - his stomach isn't large enough to accommodate the nutrition he needs.
The babies who are great sleepers that early are an exception vs. the norm. He's getting ready to go through a HUGE developmental/growth phase, and his sleeping patterns will change with it.
Sleep deprivation is one of the sacrifices of becoming a mom that, unfortunately, you just get used to as time goes by.