You should probably start by stretching his A time. I see his first A time is 2 hours. Try stretching it to 2 hours 15 minutes and see if he will give you a longer nap. Also, when my daughter does a 45 minute nap and wakes happy, it means she got a rested nap and will do her regular amount of A time until the next nap. It took me a bit to figure this out and I too was wondering why I kept getting all these short naps from her!
So, try for a 1st nap around 8:45am for a few days. If you can consistently get a longer nap, then stick with that new time. The timing for the second nap depends on the length of the first nap. If he gives you a long first nap, I would go with 2 hours 30 minutes of A time, unless he's showing obvious tired signs. You may need to stretch his times a little slower so as not to get him OT. If his first nap is still only 45 minutes but he seems rested, perhaps try for around 2 hours A time unless he's showing tired signs.
I know you say he shows signs after only an hour of being awake. Are you sure these signs are from being tired or could he just be bored? My daughter will yawn every once in awhile, but I know that's not a sign for her. From her I look for slightly droopy eyes and blowing raspberries. Very non-evident sleep cues and it took me a LONG time to figure this out about her.
A 45 minute nap could also mean OT, but if he's sleeping decently at night and not waking early I think your best bet is to try extending his awake time.
