My 12-year old son's foreskin is only retracting so you can see part of the glans Is that normal?

no. i would seek a doctor.

That is not necessarily a problem, He is now reaching an age however where he will probably want it to move back all the way and so long as there is no adhesion between the foreskin and the head of the penis he should be able to literally stretch the foreskin to the point where he will be able to pull it right back, go to the top of this page and ask " how do you stretch your foreskin" and you will find a lot of info on it.