Hello and welcome to BW forum:)
I am not suprised that the best place to fall asleep is in mama's arms
Are you?
Unfortunately, our LO grow up and our backs hurt more and more:/
I feel like what you are going through is perfectly normal at this age. They tend to try us out to see if it is pssible to achieve what they want. This is also the time to explain to them, that we are there for them but they need to learn how to sleep by themselves.
If he slept in your arms for 3 or more days already, he liked it. How responsive he is to your words? Talk to him all the time you are putting him down.
In calm words explain that it is time to sleep and he needs to lay down.
My DS was standing up every time I put him down, so I was coming in, putting him down and talking to him some more. I wasn't sure how much he gets, but my voice reassured him I am there.
Please, look through the link belov. It has some great info:)
Getting back on track using Walk In/Walk Out (WI/WO)