Query: Did he improve to 100 or did he get there during the offseason? What's his WE?
The glitch is that offseason ratings losses aren't coupled with a reset of the improvement rate in that category. The game was designed to have growth very slow at the extreme ends of either the scale or, once potential was added, the player's potential in that category. Offseason losses don't seem to reset the rate. This ends up most apparent in Stamina (like dacj said).
For instance, with a cap of 90 and a current rating of 70 a player might improve 1 pt for every 100 minutes of practice (pulling numbers out of my butt). It was intended, once the current rating is 88 (still with a cap of 90), that the growth rate will slow to 1 pt for every 200 minutes of practice. However, when that same recruit loses 3 points in the offseason (down to 85), the growth rate remains at the 1 pt/200 rate, but should have increased to, say, a 1 pt/ 150 minute rate with 5 points of remaining potential.
That's my understanding anyhow. From my limited knowledge of programming, this might not be hard to fix, but risks being a nightmare to debug. You should just jack up his PER minutes to 50 or 60 and see what happens (if everything else is at the cap).