Waking too early - wake-to-sleep won't work, any other suggestions?

[Tinkjon]

DS (15 months) is waking up far too early in the mornings - about 5am. Obviously that's enough sleep fro some babies but it's nowhere near enough for him, he is grumpy all day and is yawning and eye-rubbing from about 10 minutes after being awake. I don't think wake-to-sleep will work for us though, as he doesn't have exactly the same sleep patterns each night (goes to bed at different times, sometimes sleeps through, sometimes doesn't) and he wakes up at different times in the morning - sometimes it's 4.50am, sometimes it's 5.30am. I've tried getting him back to sleep with the usual methods but I just can't. He is just awake and ready to go downstairs (chatting etc.) but he is still exhausted and yawning.

is he waking hungry?
do you have to give him milk on waking?
is the central heating coming on waking him up? making him too hot?

DS has a habit of waking early then coming into bed with us and falling asleep again - worth a try? He's a bit older (2) but I remember him going through a phase of waking uo at 4:30/5 at around 18 mo, he seemed to grow out of it.

I think he is probably hungry, yes - but if we give in and go downstairs he's not yelling for milk or anything, he can easily wait another hour or so. I'm reluctant to feed him then because we've just cut out his habit-bottles throughout the night. It's not the heating, no (good idea though!) I have tried bringing him into bed with us but (a) I don't really want to as we've only just got him staying in his own bed throughout the night and (b) we tried it this morning anyway but it made no difference - he was just lying there, saying "downstairs!" and beeping my nose

No advice but I'm in exactly the same position and with no idea what to do!

try giving him some weetabix or something before he goes to bed. it does help to avoid that early morning hungry-wakening.

could you leave a beaker/bottle of water for him so if he wakes thirsty he could have some?

is there anything for him to do in his cot to distract him for a bit/let him fall back to sleep? a book/favourite toy - put it in when you go to bed?

DD1 was an early waker for years, she is now 4.9 and has stopped, but she used to wake up demanding milk and was definitely hungry, then tired an hour or so later, v annoying (and exhausting).

If nothing works they do grow out of it eventually...