Hi it could be SR but sorry to say I'd think his day needs extending too, he's prob a little UT at BT and if is SR then they need to be tired to get to sleep. Can you not push BT to 7pm for a while to see if it helps the messing about?
I guess you could cap his nap but then they really still need a decent nap, guessing you don't want to wake him at 5.30 am to add the extra A time on that way!? It's a bit tricky as cutting his day sleep might make him OT as he's not rested enough for the rest of his day!?