My dd (almost 6 months old) takes up to 5 oz. at a time, but I know the amount varies greatly from baby to baby. If you've got a bunch stored up, that's good, because obviously you want to have more than you think you'll need available!
Is it stored in small increments or big ones? If small, you could thaw out the amount you think he'll need and then he can always thaw more if needed, maybe?
(happy birthday too!)