when he naps, do you leave him to wake up naturally, or can you wake him after a while?
if you wake him, then you could try waking him 5 mins earlier, etc until you reach a balance where you feel he might sleep at night too.
I feel for you - dd1 has recently started ebing terrified at night, and not going to sleep (unheard of for her - she has slept through since she was 7 weeks old, she's now 4.10, an other than nights where she has been ill, has never been bothered before), and it quickly escalated to the point where dh or I had to be in the roo mwith her for her to settle to sleep (despite me assuring everyone on here that that wouldn't help )
it is a nightmare, as, like you, I have stuff that needs doing all evening, and I haveto be on the floor pretending to sleep until whenever dd1 decides to go to sleep (usually around 11pm). dh and I are doing the slow withdrawal method - basically, we went with dd1's wishes for a couple of nights, to get her calm, and since then, we have been pushing to the boundary of her comfort zone each evening. the idea is that we stop short of her panicking (would defeat the object) but that we don't let her get complacent, either (as it is not a situation we want to continue)
so each night we moved closer to the door, wihtout fuss or bother, just moved the cushions we were lying on further away form dd1's bed, and then settled down as normal. we are now outside the door (door open, another first for dd1), nearly out of sight. we stay at each stage of movement until she is fully comfortable with where we are, and has stopped coming to check that we are there, then move a little further away the next night.
the long term plan is that we move out of sight completely, with thte door open, and then start working on closing the door, again a little by little (with us still outside but out of sight.
it will take time, but dh & I feel better that there is even a plan in place, and dd1 has so far tolerated our movements well (it is only 5 days since we started moving away form her bedside, and we are outside the door already)
the general idea is to get dd1 used to self-settling. that's why we edge away, pushing her close to her comfort zone boundary(but not beyond) - so that she is alittel uneasy, but not enought to panic over, and then she learns that it is ok to be uneasy, and that she can calm herself. small steps is key to this, I think.
we have not (yet) ha an issue with dd1 panicking int he middle of the night, but I imagine we would use a similar technique to settle (keeping the same movement line impotant I would think, to keep the level of self settling consistant)
it will be disruptive, and it won't be cured overnight, but would having even an outline of a plan help you cope a bit better? I also think it is important that you and dh take turns in dealing with your ds (i know you said in your OP that your dh does) - it gives both of you a break (at times), and it also doesn't allow your ds to get too fixated on one person being there at all times (I know you have had an issue with this before, as have i with dd1)