Hun I have been where you are. Anna is totally right, if you don't extend the A times to something near appropriate for his age, he will continue to either fight the naps and not sleep or have an UT 40mims ones like you were having before. UT can cause just as many issues as OT. If he does take an UT nap or two, he will be OT by the end of the day and this won't help you much either.
I too was dubious about extending so far when I thought I should watch her cues. But it did work for me and we finally had a long napping bubba with great night sleep again. Until it was time to extend the A time again of course
Not saying it wasn't easy, DD used to give me all the sleepy cues early, but I had to ignore them a little and watch the clock too at this age. They can give you all those cues if they're hungry or bored or just want a change of scenery.
You will actually get the best odds of getting a nap near to his recommended A time hun, not when he's rubbing his face etc as just the change of scenery of going into the bedroom will give him a new lease of life and fight the nap yk?
Another thing you could try is w2s. I got great results with this,
but it must be done close to their a time or else it won't work. I would go in at 30mims after he's fallen asleep for his nap, rouse him ever so slightly until you hear his breathing pattern change...you can do this by stroking his cheek or a hand on his shoulder and this should help see him over to the next sleep cycle. I'll post a link about this for you now. Hth x
How to address habitual wakings (w2s and other methods)