I've had him for 4-5 seasons in my Newcastle save (don't play on it anymore though) and he does develop quite nicely. His technical stats seem to reach their peak quite early and doesn't improve that much more, at least in my save, but his creativity, flair, off the ball and strength rocketed for me. I think, once in his prime, he can easily reach 20 in both creativity and decisions and around 18 in most of his important technical attributes and off the ball, flair and composure depending on how you train him.
Important to give him a lot of playtime. When he was out on loan at West Brom in my third season (after I had him tutored and such in my club) he grew very quickly and had a lot of good matches. I'll see if I still have the save, but anyway - he does have the potential to become a good Premiere League playmaker or inside forward.
great, great player!