They’ve already banned him, a whole large number of them.
Why not if he wants to be able to have a place he can use to communicate without concern that whoever hes curently using will ban him?
The pressure on any site that allows him, or lets him back, willbe enormos to ban him.
Besides we can use a site that is not a societal censor, a self appointed determiner of truth and falsehood, that no one ever who has signed up for those sites ever wanted them to be their personal conscience or decider of what they can and cannot post.
The actual behavior will be to "allow" him back on the premise that it is far easier to use an already-established platform. Then they will ban him again just before the next election so he does not have time to form his own site.
Don't go back Mr. President. Build your own site. And take half of their advertising revenue with you. I pay for access to sites which also have ads, and I would gladly pay for yours.