You made a big mistake with him: you should first give all the necessary positions and SA, and only after that improve player's quality. The more stars he has, the more difficult the training is.
7* is pretty much enough, though you have to have the whole team of 7* if you farm 30 green packs every day. To achieve that you have to buy 18 y/o fast-trainers, 2-4 players every season. In a couple of season you will have an awesome team.
You should probably read some guides:
https://forum.topeleven.com/tutorial...s-players.html -- outdated, not always correct, but still very decent guide
https://forum.topeleven.com/tutorial...tml#post500463 -- simply the best summary on the forum at this moment, 'nuff said.