I think LO is OT (overtired). At 6 weeks, your baby can barely handle 60 minutes of awake time. Awake time is from the moment they wake up until the second they fall asleep. So you have to feed, diaper, wind down, pat/shush, all within an hour! I suspect since you're saying 30-40 minutes AFTER eating that you may be waiting too long to put him down for nap.
I'd go by the clock a bit for a little while. If we're aiming for 60 minutes of A time, then we start our wind down procedures (changing diaper, rocking or singing, etc.) 10 minutes before, or 50 minutes after they woke up. Let's make sure you have a nice predictable wind down routine. That's one of the most valuable things you can do at this tender age!
Keep working on the shush/pat. It's totally predictable that he won't get it at first! And in fact, it may take a good month or more for him to be able to fall asleep completely on his own without your help. BUT, if we can time nap better you should see him sleep a good 1.5-2 hours without any additional help from you!
Give that a try for a couple of days and report back, and we'll work on it some more!
If he wakes early from a nap, you can try to shush/pat him back to sleep for 15-20 minutes. If that doesn't work, then you feed and put back down for another nap 15 minutes later or so.