It's his pick until he offers the master to you. If you truly are only "friends". Your having more stuff is not his fault, and how about his (likely) being bigger and needing more room just to move around and maybe for his stuff too. At least let him start that conversation.

Does the master bedroom have its own bathroom? Is it larger than the other bedroom?

In roommate situations where the bedrooms have been unequal, the solution I've seen is that the person who gets the "better" bedroom, be it larger or sunnier or with its own bath, pays a larger share of the rent.

One roommate pair I know switched in and out of the better bedroom every year, which worked for them, but would have been a huge hassle and not worth it for me.

Why have neither of you brought this issue up? It's a natural question. So ask him if you can have the master bedroom and offer to pay more rent and see what he says. And if he says he wants the master bedroom, accept it gracefully. And maybe negotiate a greater share of the common space to store your stuff.

Share the master bedroom and use the second bedroom for guests.

Just talk to him, he will let you have the master room

This situation won't last long if you can't clarify it with your friend before you move in. If you have more stuff, maybe you will need to simplify. And just maybe he does have more belongings than you. Also, do you expect this to a long term situation?

My son moved into my apartment to help out after I had several major surgeries. I pay most of the rent, but he had the larger room since he has more stuff, and I'm glad it's in his room instead of cluttering up the whole apartment.

Save problems if you clarify the ground rules first.