time, sorry, nothing else works ime. Give him a week, try to get him into bright daylight as much as possible in daytime & in darkness when he's supposed to be asleep. We get it bad both ways every time, count yourself lucky!!
Very dark bedroom at night. Lots of running round outside in the day. You don't say how many daytime naps he would usually be having but I'd suggest you don't let these go over 2 hours (in one nap). It'll take a few days. Schoolboy error of going to bed at 11am but I know how tempting it is!!