I am just not seeing that much risk here. Cosigning on a loan is an entirely different story. It's not like he even wants to share a bank account with you. He will be an authorized user on an account to which he has no access.
Is it possible that he will agree to shred his card then steal yours or call the issuer and try to get another one sent to him? Of course, it is possible. If you can see him doing this, then you should absolutely not add him as an AU (you should also probably rethink being with him at all). Ultimately this decision will come down to how much you trust him. If you do trust him - and I assume you would to be considering this in the first place - then go ahead and add him as an AU, but watch your account (which you should be doing anyway). If he ends up using the account behind your back anyway, then you've found out he isn't boyfriend material and it's time to, as Dan Savage would say, DTMFA
Either way, it's been mentioned that being an AU won't help him all that much anyway. If you agree to it, he will get a little boost but he will still be better off getting something on his own on top of that. He can make his way on his own if he has to, and you shouldn't do it if you feel even the slightest bit uncomfortable about it.