At this age he may well still be cutting some new teeth. There might also be a hidden ear infection with no other symptoms - some people notice it more when they're lying down, and both mine (and myself!) have turned out to have ears full of fluid with no other symptoms that I knew about but felt lots better with treatment. Is there something new in his diet that might be giving him a tummy-ache in the night? Dairy and gluten are often culprits here and can even cause some reflux-like symptoms in some people, which would be a bigger problem when he's laying down. Has he been on antibiotics recently? That can cause some major tummy trouble until the gut flora is restored; probiotics are your best bet there.
I would try some pain meds before bedtime and see if that makes any difference. If he sleeps better with meds, you know that pain is likely the culprit ad you can investigate teeth or get his ears looked at, or check the diet and see what's new in there. But I don't know that I'd do much in the way of sleep training till I've ruled out a physical cause that might be causing pain.
