Baby will only sleep on boppy pillow!?!?

The last week my 5 week old won’t sleep at night unless he is swaddled and paying on his boppy pillow. I have been making sure he is well propped and move him up when he slides down. However I know this is unsafe and it makes me extremely nervous, but everytime I remove it when he falls asleep he wakes up within 20 minutes and won’t go back to sleep until I put him back on it. With using it he will sleep a solid 4-6 hours at a time. I don’t know how to get him from being so comfortable using this. I may also add that he is currently 12 lbs so he fits real snug propped up on the pillow. Please don’t come after me for this. I have a king size bed and he’s also just out of arms reach on my bed when he sleeps and I make sure there’s no blankets or anything around him.