You just need to be more stubborn.
That strictness and sternness follows for silly bedtime requests too. He is old enough to understand that the answer is no to all of these requests. Make extra special care and attention at bedtime to ensure all his needs are met (and verbilise this - tell him as you're doing things so he can't then, for example, moan about his pillow).
Given your recent relationship break up, I can understand you maybe not being in the best place to handle the conflict right now. You don't have to leave him screaming and crying. Especially if that triggers you.
If I was in your position then I'd agree to staying in his room while he goes to sleep (at least for a few weeks) - but only if he follows your rules.
Set your results in a simple way. You expectations should be:
- He lies in bed
- He is silent
Then develop a mantra that you repeat over and over again. "Sleep time now. You must lie down quietly. Nan night".
Then don't tolerate anything other than lying down and being quiet. Any slight movement towards getting up - stop him, repeat mantra, lie him back down. And noise at all (including talking/asking stuff), reitterate the need to be quiet with the mantra.
You can still stay there and cuddle if needed and be kind with him, especially if he gets upset. But tolerate ABSOLUTELY NONE of his demands or messing around. His only option should be lying down and being silent.
Then over the course of a few weeks, be slightly further away from the bed as he settles to sleep. But continue to accept no demands or messing around.