Hi yujai, as Marcia says, well done for holding out for so long!
We did long am, short pm nap so I might be able to help with that. We didn't do it the other way round. Marcia has already given you fantastic advice. I wouldn't go cold turkey to one nap, but definitely start working towards the transition.
There's two ways you could do it, either leave the first nap where it is and cap it at 45mins, then do a longer afternoon nap. You might need to reduce the A slightly after this short nap so he doesn't get OT before his pm nap. Just like you have been doing already.
The second is to do as Marcia said and push the morning A by 15mins every few days to lengthen the morning nap. Then look at doing a shorter 45mins CN in the afternoon. With this version, they need a shorter A to bed than you have been doing because of the CN.
It's up to you or him which way you'd prefer to do it. Your day is already long enough, but he is getting a lot of day sleep for a 16mth old so I think capping one of the naps will eventually lead to better night sleep.
And lauradj.. I take notes too! Or look at my past posts on here!!! Yep, I'd lengthen that A, hold for a couple of days then lengthen again if needs be. Do whatever you can to get that decent nap in the morning. Might need to cap the other nap to stop the day getting too long.
We're going to have to have our first intentional 2 nap day for a looong time! Back to a 5.30am wu this morning

. Need to break this before it becomes a habit again. I reduced the first A for a couple of days thinking she was OT, but that has led to a much shorter nap and too long of an A to bed for her causing early NW. I hope one day of 2 naps will take us to a better BT and wu then I can push out the nap to where it used to be to get back on track. We were doing so well for so long, it's just sooo disappointing.