Your problem most probably lies in the CA. if you add a bunch of stat points without raising the CA, his stat points will start to fall off fairly quickly. If you raise his CA along with the stats it should eliminate that problem. If he still drops stat points, you didn't raise it enough, if on the other hand all his stats start increasing in training, you raised it too much.
I take it, from the responses here, that there is no limit on the total number of attribute points based on the CA. I was trying to find one, so I could create a player without wasting any attributes. I've found when setting the attributes below what is appropriate for the CA ends up wasting points which could be used elsewhere, and going over the limit seems to take the points from the best attributes, rather than randomly. It seems like there's a limit based on the CA, but I'm not sure what it is, or if anybody even knows. I wouldn't mind knowing it's a small range, as losing or wasting a few attribute points is better than losing or wasting loads of them.