Hi there, have you tried pat/shh instead of pu/pd.
It may be helpful to try that to get your lo used to the idea of independant sleep, and then when he is older try pu/pd.
My dd is about 22weeks, and we started her on pat/shh.
I would swaddle her, hold her in my arms facing me, she had a hand out as she likes to suck her fingers. I would do pay/shh until she became drowzy enough to not really notice I had put her down. She was still awake, but she was almost gone IYKWIM.
Good luck hun, keep us posted it, and ask more questions if you need to.