Okay - if he is having really long naps in the middle of the day then there is a significant possibility that it is robbing from his night sleep. I'm not sure that I would expect much more than about 12.5-13h total sleep in 24 from the age of 2 (not sure exactly how old your lo is - obviously the older he gets, the more likely it is that he needs less sleep). So if he has a 3-4h nap during the day, he only really has the capacity for another 9-10h night sleep - which seems to be what you are experiencing.
One of my los is very high sleep needs and I would still never have let her sleep more than 2h by the time she turned 2 - because even a high sleep needs lo is going to struggle to sleep well at night with too long a nap during the day.
So to start with, I would say that you need to cap the nap at 2h to see how you go with that. I think that by this age you can get away with setting his nap time - given that you are going to struggle to do an early bedtime on your own, you don't want to be bringing it too early and giving him a very long afternoon (which is likely to lead to OT even if he is getting enough sleep in the 24h period). I would work back from his bedtime - which I think you say basically has to be 8pm? - and have his nap end at that point (which I think would be somewhere between 2 and 3pm - 5.5-6h A is fairly typical for this age). So again working backwards from that, his nap would have to start between 12 and 1pm - I think that he is likely to get back on track more quickly if you try to avoid moving his nap too early in the day to compensate for his EWs.