if you're serious about it, you'll need to compensate him for what he's going to lose out on (ie telly, milk).
So, talk about how from it's all changing from now on bcs he's getting up too early and he's grumpy all day. Explain that now he's a big boy he has to stay in bed until he hears you getting up, say (or you'll go into him or whatever).
Star charts would be useful here. Let him pick the stickers. Let him help draw up the chart. Involve him in what's happening.
Also, let him pick a new soft toy, explain it's the stay-asleep bear or whatever, it'll help him to sleep in like a big boy, it needs lots of sleep. If he wakes up too early and makes noise, he won't get to keep it.
Give him strategies for when he wakes in the night -- ie, accept he's going to do this, and show him how to cope. Ie if you wake up, use the potty if necc, turn over in bed, stroke sleep toy etc. Give him books to look at, as a last resort.
But don't crack. If he wakes up and demands attention, I'd do rapid return until it sinks in.
And if he manages it, make such a celebration out of it, ring grandparents ec.
Could it be wee related, do you think? Does he have a potty in his room?