Yes, he might be getting a little bit OT. Often LOs need a CN still every few days to stop OT building up, so that's probably what's happening here. How long is it since he last cat napped? You might need to do 2 naps for a day or two, then a 3 nap day, then a couple more 2 nap days before another 3 nap day, depending on how he's doing, when the naps fall etc.
I would try offering a 30 minute CN at 4.30 or 5ish for a 7pm bedtime and see how that goes. If you have a fairly set time for the CN then you can try and AP up until 4.30pm, say, and if he's tired overall he might go for it, and if he's not then he'll skip it and you can do an EBT instead. That way he can self regulate and you'll probably find that some days he's cat napping and others he isn't according to how tired he is, rather than trying to plan the number of days he goes between cat naps.
And don't worry about not being able to read your LOs cues - my two were really hard to read as babies as well! Sometimes you just have to go by A rimes and nap lengths and make your best guess
The good news is that my two got much easier to read as toddlers...
