I don't think he is readyfor a 2-1 Nap transition
The readiness can change very quickly. Some LOs go on a journey of transition over months and months where you might find semi-stable routine in there and keep tweaking it a bit when needed until eventually the drop to 1 nap. Others though seem great on 2 naps and then quite suddenly not great and need to make the move to 1 nap early. it's a possibility. Keep an open mind
I'm wondering if i need to cap his morning nap at 90mins?
Yes this is an option, it's only going to work if there is no resistance for nap 2 when he does 1hr or 90 min nap here at nap 1.
You can certainly try this. Then do nap 2 and BT at the normal times.
Some LOs continue on two naps of around 1hr each for a good while and seem fine on it even though we usually say that a nap is not restorative unless it is 1.5 hrs or longer.
The difficulty I have in this routine is that LO learns not to sleep longer but to wake at 1hr so when they have to drop to one nap you may need to re-teach that the nap needs to be longer, 2hrs or more.
Another option is to reduce nap 1. Start it later and end it at the same time, so instead of 3-4 do 3.30 - 4pm. I see you said you tried this and he still resisted, it could be that you need to be consistent over a number of days. It could be that you tried to WD too early so he started his resistance and then continued it well beyond a time when he might have gone down more happily.
I would be willing to move that CN later at 4 - 4.30 and move BT 30 min later to 8pm too.
it's just another option really so you have some idea where you might be headed and can have a think about what you feel will suit LO better.
Today, he fought his nap until 16:15 at which point I gave up and powered through without one and have done an early BT of 18:30 instead.
LOs can often go through a phase of refusal and then suddenly decide they do need that nap after all and return to accepting it more readily. I wouldn't read too much into it happening only one day but if this continues and you just cannot get him down for the CN then I'd suggest a bigger change to the routine for example:
- cap nap 1 at 1hr (so 2 naps of 1hr each say)
or
- push that single nap later even though he is very tired to balance out the day and reduce that long A time before EBT
or
- a mix of 1 nap and 2 nap days. Having a 1 nap day can mean LO is happier to go down for 2 naps the following day. Personally I don't like this route as it just doesn't suit mine (or my DS's) ned for consistency of routine. But it works for many LOs.
Hope this helps