Porridge, banana or oatcakes with nut butter.
What is his food/milk intake like during the day? Is there any chance he's just filling up on milk during the day and not being hungry at bedtime, or that dinner is too soon before bed? I think the bedtime milk is usually the last to go for most, so could just be a weird phase.
That said, if he's happy to have a snack instead, something with a good mix of carbs and protein is probably best.