I bring this up any time someone is leaving, and even when I have left places. You should review all access, change all passwords he may have had, revoke all access that he had.....and you do this....as much for his benefit as for yours.
He is leaving, he is naturally the person who is going to be blamed, either directly (he did it) or indirectly (Oh he used to do that, and hes gone now....). Thats normal, and some amount of it is fine. However, you owe it to yourself AND to him to be sure there is no question that, if there is an incident, there is no reason to suspect him.
Nobody wants to be in the situation where there was a compromise after an employee left, and now there are questions. If there is an incident there will be questions either way, but the only person who benefits from his still having access is the attacker whose actions are the problem.
I felt better knowing my access has been fully revoked and there was little chance of there being questions later.