Hi,
Thanks for all your suggestions. But in the end there are still so many options and I'm confused again. I think I will play him DC as his natural position says. And yes, thanks to all your opinions I realize that he can't play as midfielder because his passing and creativity is not good enough and will never be.
About his speed: some opinion says he's not good enough to be a striker to get past opponent defenders, but it's the same for defender not fast enough to follow opponent striker. However I note that his stats will be increasing dramatically because we have best training facility, best coaches and best tutors for him to learn. I would be happy if his acceleration reach 16 and pace about 18 at peak. That's more than enough for a strong striker and pacey defender.
About his strength/stamina: I will be very happy if these number reaches 15/17 for him.
His attacking skills are good, however, so I think I will let him take freekicks, and then goes back to defend. His heading, finishing are good also, so he will come forward at corner kicks to harass the defenders and gk
I think there will be large chance for his heading/tackling/marking to improve more than 16. As someone said, I would probably train him to become a DRC, but when looking back at his off the ball/dribbling, I would think twice. Maybe DC best suits as the game says for himself, a natural defender.
In general he will be a good versatile player for my middle line. Thanks for all your suggestions.