I'm not sure if you've been overplaying him, have you been starting him every single game? I would use a rotation with him on my team, so although I used him probably more than any other fullback he would still only start about 32 games per season or so.
Now, in 2016, he is ridiculously good, averaging 7.6 currently and I keep getting offers that teams want to buy him for 35m+
I've been individually training him on positioning since the start (now at 15) and his schedule is more speed oriented, his speed is 17. I wouldn't be training him individually on passing personally, that may have something to do with his decreases in other areas
You probably already know this, but make sure to reprimand him for not training hard enough in private chats whenever you see decreases