Hi S.!
My daughter went through the exact same thing. She's 20 months now. She was sleeping exactly like your son. 2 naps lasting 1.5 hours, it was great! She started having trouble with the second nap like you son. Wouldn't go to sleep, just played. I did what my friends did and tried the one nap thing. kept her awake till 1:30pm and put her down. She still got tired around 12ish like before, but we just kept going, running errands, eating lunch, etc.. She would perk up again. Then around 1:30 I put her to bed. She slept 3 hours! Sounds good, right? THe next day I did the same thing and she only slept 1.5hours. I was like, "what the heck?" The next day I did it again, and it was like a 2 hour nap. So sometimes its really long, sometimes its short, like 1.5-2 hours. If she sleeps only 1.5hours, I let her stay in bed and play for 30min. I need my alone time!!!! Sometimes we run errands and I don't get her down till 2pm or so and she sleeps fine. At first I was bummed about only having one nap, but now its great because we can do so much more! Play dates in the mornings, lunch, run errands, then she naps and I can do things around the house. She wakes up around 4 and then we have more time to go to the store if we need to with plenty of time to get dinner ready. We eat around 6:30pm and she's in bed by 8:30 or 9. She will sleep till 8:30 or 9am. Its great! Please let me know if you have any more questions
love, S.