We had this after going through some changes too so we swapped to a more strict nap schedule in the day instead of just letting him sleep whenever he's tired. It did help for a couple of days but then he was back to having a split night last night, not sure why either.
@Sarah I agree with this! I only let my girls nap for two hours during the day. Last nap depends on how their day naps go. They like to try to go to bed around 5 so I let that happen but only until 7 and then in bed by 8:30 (it takes a whole hour or two to get them to bed)
Whats his last wake window before bed?