Hi there and welcome to BW.
Yes, you are correct in knowing that the A time is likely the culprit. You are looking at 2.75-3 hours of A time at 7 months. Seeing as you did not see results with a 10 minute increase, I may suggest jumping to adding 30 minutes. Yes, it will be a struggle, just keep in mind that his little body is stuck at the 2 hour A time and will need the push to extend. Just keep the activity low key... read, sing, slow dance, etc. Sometime the "stuckness" is boredom and he needs something new and different to do.
His middle A time is quite long (due to that short nap) and maybe you can get those A times evened out a bit. You may find that in the very near future, with the A time increasing, that cat nap at the end of the day drops. Many drop that around 6 months.
What do you think?