I'd say most students can improve a grade or 2 from their first year to final grade
Offers are made on predicted grades.
His offers won't take a potential massive grade improvement into account.
If he's getting Cs and Bs in Lower 6th offers will be made based on the assumption he will get those grades as his final result.
It's vanishingly unlikely that an institution that's entry requirements are AAA will make an offer to a student who is predicted BBB. The likliehood is he won't even be invited to interview, sorry. They need a broadly similar level of academic ability in the cohort.
If he somehow miraculously got AAA on results day and his heart was set on say, Bristol, his best bet would be to take a year out and apply with his AAA already in place.
Also an offer is just that, an offer not a place. Unless, and these are quite rare, he got an unconditional offer.
Otherwise he'd have to at least come close to the grades the offer is based on.
If his offer is BBB and he gets CCC, chances are he will lose his place at his Firm.
If he gets BBC they might still take him, a chat would most likely be involved.
It's wise to make sure the Insurance offer is lower.
Firm should be based on the most realistic best possible outcome.
Insurance based on a worse than expected outcome.