How about a night light on a timer, set it so it turns on when he is allowed to get up? Like the Gro Clock but much cheaper
Make a huge deal out of his new light.
If you think he may be scared perhaps the light could stay on and he can get up when it goes off.
Another idea is to get him a toy, tell him a story about how the toy keeps getting up at night for a drink/cuddle etc and ask if he could look after the toy and make sure the toy stays in bed with him all night.