Too tired for afternoon activities, won't sleep

[Erobin19]

Recently my 21 month old has been extremely tired during the day and he's become difficult to work with after 3pm. If he doesn't have 3-4 hours sleep in the morning/early afternoon, which he very rarely does, he gets tired at about 3-4pm and won't play anything constructive with me (won't sit and read or follow any kid of instruction, even for dancing games). He would happily just sit and watch TV until dinner at 6 30, or he'll just play quietly with his cars in a way that I find difficult to join in with (shouting Duggee every 2 minutes). He really needs to go back to 2 naps a day but he'll be laid in bed for over an hour and be no closer to sleep than before. I don't necessarily need to join in with his play, unless he wants me to, I'd just like him to do something other than watch TV 🤣😭
He'll happily run around but that obviously makes him even more tired, and an afternoon walk can be the most stressful thing when he's like this. He doesn't get grouchy, just if you try to touch him in anyway he'll just get a stiff back or lump legs with a smile on his face and won't listen.

Any activity ideas?

NB I'm not worried at all about the tiredness being a health issue, he's not lethargic, just getting up a bit earlier in the morning than usual making him tired earlier.

21 months old and you want them to have 2 naps? Mine started dropping naps all altogether at 22 months.
I suggest you plan all the active things in the first part of the day and have less expectations for the afternoon. Fingerpaints in the shower is my rescue tool for a tired toddler that keeps her entertained for good 45 minutes.

This is a stage most kids go through when they drop a nap. You just have to ride it out. Let him play with his cars/watch TV for the time being and he'll soon get used to the lack of afternoon nap and afternoon energy levels will rise.

Two naps at this age is a lot. Does he sleep well at night? I'd also say 6:30pm is very late for dinner at this age. Maybe it's time to drop the afternoon nap and move dinner and bedtime forward. That will reduce the amount of time he's tired but not in bed (dinner at 5pm in bed by 6:30/7pm if he's tired?).

I dont WANT him to have a second nap, he dropped the second nap before he turned 1, he just really seems to need it atm. We don't get home from work/nursery until 6 45 at the earliest, then it's dinner ASAP and into bed by 8 30 at the latest on a school day. I don't like changing his routine too much so when we're not in work he's usually in bed by 7 30. He was only having 1.5 hour nap during the day for months but in the past couple of months he's just been incredibly over tired. He has a good 2.5 hours or more in nursery but I can't get him to sleep more than 1.5 hours at home.
He needs to wake up later tbh, he's been getting up at 5 30 - 6 30 then napping at 11 (instead of, say, 1) but I think he's just got used to it because we have to get up at 6 on a school day anyway. Which is bizarre in itself since he was an 8 30 kinda guy until Christmas, and then one day he just woke up at half 6 and hasn't had more than a handful of later mornings since 😭

Do you have a DH? Can you stagger your work times so one of you can pick him up earlier (and the other can try to get home in time to help with bedtime routine)? Eg if you each have a 1 hour commute and need to work 8 hrs - one leaves at 7 to work 8-4.30, pick up DS 5.30, dinner 6.30 bed 7. Other leaves 8, drop off DS, work 9-5.30, home at 6.30.