((((hugs)))) I dunno, that's really hard. I think he's still overtired, because that's really not a lot of nap, and a 14 hr or more day. Plus the canines and plus the 18month sleep regression (which is a doozy). How come you decided not to do an early bedtime, was it because he had a second nap?
I'd probably keep him up til 10 and then hope and pray for a longer nap. He needs to be asleep in bed by 6 at the latest tonight, you could even do 5.30.
I know you know this but if you're doing WIWO, you need to keep up with it. Or do something else to keep him quiet in his bedroom (cuddles/whatever). I do think it's really important that he doesn't get to start his day and have breakfast until a reasonable hour.
More hugs to you though. I remember those days so well, I remember raging at CBeebies that they didn't start broadcasting til 6am! 6am??!? I had been up with a mardy toddler for two hours by then!