We have a close friend with the identical problem. They have had several sleep specialist consultations and apparently it's not unusual at this age to develop night separation anxiety and even if they aren't displaying outward signs of separation anxiety they don't like to sleep because they are away from you.
They are now working on lots of reassurance and she is allowed to come into their bedroom to "check they are still there" whenever she wants during the night (at least that way it is 30 seconds wake for them rather than hours) and it is slowly improving.

It's just something to consider - it might not be a sleep issue as such. Have you tried asking him why he is awake?