I've had a boxer continue growing past 14 months - so it does happen. But at the same time, you'd have to say that if he hasn't finished growing, he must be very close to it (so it would be surprising if he snuck in more than another centimetre or two). Can you recall how long ago his growth plates disappeared from view? That might help to estimate whether or not he is likely to continue to grow.
22 inches is within the height range specified by the boxer standard for males, so its not as though he's undersized or anything. He could probably stand to be a lb or 2 lighter for that height though