How old is he?
DD2 had 20min cns right around 5pm before she dropped to 1 nap, so I would say that a cn from 4:45-5:15 should be ok. You can always cut it to 20min if you think it is a problem.
The day he had the mega long nap, did he seem tired at bedtime? Both my LOs did longer A before bed, so they were doing 4.5hrs A in the am, big nap, then 5.5hrs A before bed.
If you want to keep the cn around then yes, you can cap that am nap at 1.5hrs