Generally speaking, average A time at 5 months is 2:15-2:30. 30min naps are usually OT but occasionally if a LO has a slightly shorter than normal sleep cycle, they can be UT.
Have you had any luck resettling?
After a good night like you've said he does, his first A time should be able to be the same length. There are babies who do better with a shorter first A time but often that's extending their night and their early night sleep is really poor. If you think about it, night sleep is most restorative, so he should be most able to stay away the 2:15-2:30 first thing in the morning.