Hello, welcome to BW forums
I'd probably try wake to sleep using the shush/pat method where you either begin shush/pat before the wake up time (so starting at say 40 min) or ready yourself so that you are right there to start shush/patting the second there is any sign of waking. This is often more successful than waiting for LO to fully wake and call out to you before trying to resettle.
If your LO always does 50 mins it could be his sleep cycle is that length so it's possible he hasn't learned to transition from one cycle to the next and the wake to sleep can help with that.