Sorry that it's taken me a bit to respond to you...
Looking at his routine and how you've described things, I think that we need to find his A time "sweet spot"
You're going to have to gently increase his A time so as not to make him overtired. I'd start with the first A time of the day, and extend by 5-10 minutes. Then after a couple of days, just keep increasing across the board.
His A time (which includes eating, activity time, and wind down--or in other words, eyes open to eyes shut) should be about 1h15 now. Granted, he might start yawning a bit before, so unless he's just a high needs sleep baby, I wouldn't put him to sleep based upon that first yawn. I know what Tracy says in her book, to have them in bed by the 3rd yawn, but my LO would start yawning even just 5 minutes after he ate...he would then refuse/fight going to sleep because in reality, he wasn't tired enough yet. He was just yawning
I think that's what's happening with your LO--that he's taking UT (undertired) naps...at least that first one. Usually 45 minutes is indicative of an UT nap. I've personally found it impossible to extend an UT nap, regardless of how much shh/pat I did. Normally after an UT nap, you have to decrease the A time and put them down for their next nap a bit earlier. If you try to make them do a full A time here, it will result in them being hard to settle, but now because they're OT (overtired). I'm thinking that he's fighting sleep and the shh/pat for both of those reasons, so, again, we just need to find his perfect A time.
So, for example, if he woke up at 6:30, ate, played with you for a bit, etc., I'd try to have him asleep no later than 7:45 (maybe 7:50 if he needs even more A time). That means, that however long he needs to get nice and settled to start drifting off, say 15 minutes, you'd want to start the wind down routine/shh/pat at 7:30.
He should calm and drift off fairly easily--maybe a little fussing as some babies do that, but he shouldn't fight it or wake up after 20 minutes. If you get a 40-50 minute nap, I'd say that was an UT nap. If you can't get him to go back to sleep, then scale back on the following A time; perhaps make it 1h long, so that he can catch up on his sleep and so that he doesn't become OT. Then, once he's got a good nap in him, try gently extending the A time again.
If you get a 30-35 minute nap, then he was probably OT, and you'll need to scale back on his A time in general, i.e. maybe do 1h10.
Give that a shot and see how things turn out. You can gauge if the A time is right by 1) how he reacts to going to sleep, and 2) what kind of nap you get.