Given that the team was pushing Jaylen's All-NBA case pretty hard at he end of the year I'd say there's a nearly 100% chance they offer the supermax. And I'd say about the same that Jaylen accepts the supermax, because nobody turns down that money.
That doesn't mean he'll be in Boston the entire contract, a year, two or three down the line he could demand a trade or the c's could move a different direction. But I think the chances he's on the team in the near term just skyrocketed.
Signing the supermax prohibits him from being traded for 1 year from date of signing, so that means if he signs he'll be in Boston AT LEAST all of next year.