I feel what you're going through...just got out of the same vicious cycle with DD2!
Can you shh/pat him back to sleep when he wakes at the 40/45min nap transition? I used to do that with DD2. Sometimes, if shh/pat didn't work, I would APOP for the 2nd half of her nap so that she could rest/sleep until the beginning of the next cycle, just to get back on track for the day.
Also, with DD2, I noticed her sleepy cues were changing/not the same at about 7 months. could this be happening with your LO?
V.