We always walk out on DS now if he's silly - he's the same as your DS sometimes. We only come back when he's "nice and still and quiet and ready for sleeping". Some nights we have to go back and forwards a bit, but eventually he settles down and goes to sleep. We are also finding that (more and more) some nights we don't have to come back at all - he goes to sleep on his own (except for a brief stint at around 5 months, that NEVER happened before 23 months so it's a big deal for us).
Have you tried something like that - sort of a half-way between staying with him while he's silly and leaving him altogether? Set ground rules for when you'll stay and when you won't.

It is super stressful being in your DH's position - so definitely understand you can't just keep going as you are.