How to cope with baby who won't nap??

[babymooner]

Does any one have any advice on how to get nap refusenik to sleep during the day?
14 month old DS has dropped morning nap (point blank refuses unless he's in his pram and falls asleep inadvertently, in which case he then refuses any further lunchtime nap and screams hysterically for an hour if I try to impose one on him). He's a tricky sleeper anyway - wakes up easily, won't sleep in car etc but lately he's been waking up from lunch time nap (has it at 11am because he can't last any longer) after 45 mins or so... he's then massively tired, goes to bed early and wakes up... earlier and earlier. 4am this morning . Worst thing is he is youngest of 3 (5 yr old DS and 3 yr old DD, who has just started kindergarton) and he's waking them up so they're knackered and cranky, and I'm knackered and cranky... I know he's waking up early because he's over tired, and I know that's why he isn't napping well, either (when he's really rested he'd been known to go 13 hours at night AND sleep for 2-3 hrs during the day... a beautiful phase that only ever lasts a few days before something like a new tooth knocks him off course again) but how to I get him to sleep to get him over the whole over-tired thing? I've done the 'watch for the signs' thing, but even when he sleeps, he wakes up after 45 mins and WILL NOT go back to sleep.
And no point putting him to bed later - then he wakes up several times in night. Super early bedtime is only thing that works (5pm - then he'll usually go through until 6am), but that kills our day, means no playdates, and now that he's refusing to nap, even a 5pm bedtime isn't helping.
And I thought third children were meant to be easy?!
I'm coming to the conclusion that there is just nothing I can do with this one except ride it out, but if anyone has any better ideas, they would be very gratefully received...