Hi there, welcome to the boards
He needs more A time
1.5-2hrs is more suitable for a baby of 3-4 months, at 6 months he probably needs a good 3hrs A time to do a decent nap. Have a look here:
Average A times- BOOKMARK ME! At this age, tired signs are less reliable - babies can get used to looking tired at a particular time, but they're not tired enough to do a good nap. It can also mean they just need a change of scene. We'd usually advise to try a new A time for 3-4 days and then judge whether it has made an improvement. If a big difference in A time is required, you'd add 10-15 minutes, then stick with that time for a couple of days, then add another 10-15 mins etc, so as to avoid over tiredness.