most banks require a SSN at the very least to open a bank account. some require a state ID as well.
you might be able to add him to a joint account without a state ID, but he will need a SSN. Which he can apply for one when he gets into the US. Just not before. He'll need to make an appointment with a SSO for that. you cannot just walk in anymore -
but in short, no he more than likely cannot open a US bank account until after he arrives and has gotten an SSN
you might need to check directly with whatever airline he will be coming over with, but should only need documentation for the cat, like updated vaccine records. but different airlines have different requirements. but its typically very easy to do.
it depends on how he got his cellphone and what kind.
if he got it directly through a carrier (not sure what its like in France) it might be locked to that carrier, and he would have to get a different one in the states.
if it is something he got secondhand or directly through the manufacturer, it might be unlocked, and can be put on any cell providers plan. my husband has an iphone that he had purchased long before he moved here on his K1, but Germany almost always has phones that are unlocked, regardless of where you get them. and we were able to just add him to a cellphone plan here, no need to do anything
depending on what kind of phone he has he should be able to just google "how to check if my phone has a carrier lock" or something like that. an iphone will say "No SIM Restrictions", which means he should be able to be put on any cell plan. it should say something similar on androids.