ditto to the other posters, you've gotten great advice so far!
Fighting the afternoon nap is a clear sign that they're asking for just one nap. Tricky part is to find out HOW and WHEN would they like it LOL, but we can work it out. I'd say to go with baby steps for nap # 1, pushing it 15 mins away each day if you find it still goes smoothly and he still goes down for it and stays put. Then the second gap between that nap and bedtime won't be so tight in case he does not take the second nap and in case he does take the second nap, you can also wake him from it at the time that you consider is ok for him to be up and hang tillb ed time. Give it a 3 day go for each 15 mins increment of the nap. That can make up for a smooth transition. When he starts having short naps or waking at night, you might want to stop and make a recollection of the actions taken and the results gotten.
It is a difficult transition for some but beleive me, some others (a few tough, but there is hope!) find it smooth. it is just about trial and error and stopping when you find a dead end.
Good luck and keep us posted on your improvements and setbacks (crossing fingers for you to find none of this)