Mir, it doesn't sound like a phase as you say - I wish I had some answers for you!

A couple of things I'm thinking of - perhaps he is overtired and is finding it hard to settle and unwind? This can in turn lead to waking (as it did with my daughter, especially after one sleep cycle) as when LOs are overtired and they finally go to sleep they tend to conk out into a really deep sleep too quickly, which then means their sleep is out of whack and they often wake from poor quality sleep.

I know if I leave things too long or have too long a bedtime routine with DD she has a tendency to hype herself up as she gets more and more tired so she can keep going. At that point it is a lot harder to settle her down for sleep. Could you try starting his bedtime routine say 15, 20, or even 30 minutes earlier? How long is your bedtime routine going for? I have read that it's important that it's not too long or too short - shorter than 15-20minutes and it's not enough time to have your baby feel like they have unwound, and longer than say 45minutes is too long as they are getting overtired at that point.

It might not be the answer, but sometimes an adjustment like this can make a difference to the quality of sleep they're getting, which can help break the cycle of waking.

Another thing I'm thinking of is that he's reliant on you to be there to go to sleep, if you are staying there until he is fast asleep. If he wakes and you're not there, he's wondering where you've gone. You could try gently and progressively trying to get up and leave him to fall asleep on his own by leaving him a little earlier each time he is falling asleep, till perhaps you are getting off the mattress when he is drowsy. That way he is aware that he's falling asleep on his own, and may be more likely to feel safe and secure about waking slightly in the night enough to go back to sleep on his own instead of waking fully wondering where mummy is. This works for us.

Finally, it could be to do with separation anxiety, especially about not wanting to settle to sleep at night to begin with - he doesn't want you to leave him so bounces about and stays awake to ensure that you stay with him. I know it's been going on a long time, but separation anxiety can be a long term thing, and it often comes to a peak over (sometimes) many months before taking time to settle back down again. I don't know that I have any answers for this one as we've been through it at night too as part of our sleeping problems, but I just tried to ride it out as I didn't know what to do. My DD's separation anxiety is a lot better, including during the day now, but we've been dealing with that since she was about 5 months old, and she's 13 months now. I still cannot leave her with anyone (nor do I want to) but that's a whole other story Perhaps you could think about reading a couple of books on gentle parenting techniques for sleep - Elizabeth Pantley's No Cry Sleep Solution or Pinky McKay's Sleeping Like a Baby. I haven't read either of these, but I have heard lots of good things about them for helping with sleep issues.

I really hope it improves for you and your DS and that you can find some answers that work for you