Are you bottle feeding? If so is he taking a bottle to sleep? If o I would say that would be it! I kept my son on the bottle till 16 months, and as soon as he came off he started to sleep longer!! Try keeping him up 30 minutes later and see if that helps!
If he used to have a bottle at night until recently, then he might be getting up to eat, which at 8 months, is not really necessary anymore, if you ask me. Not if he is full when he goes to bed anyway. Offer him some water and put him back down. It might take a few days, but he will get the message.
Thanks. He is usually breastfeed but I have recently tried to have my husband give him a bottle to see if that changed anything. He nurses (and not always a full nursing) around 7:30, so I feel that one feeding during the night is ok since I'm not absolutely sure he ate enough. Sometimes I'll just pat him and he goes right out.