is there a loop on the cap of the head? something that the S-hook could hook onto?
That's how I remember my AOD being...
IF that's the case, get a pair of hemostats (you can get them from the walmart fishing section for 3-5$), set your dolls head aside, pull up the shook up - stretching the elastic out a bit, and clamp the string at the base of the neck (so now you have a length of elastic sticking up.
slide the head on, and then hook the s hook on the headcap before carefully removing the hemostats (while holding the head/headback together firmly (so the head doesn't snap back on from the tension) and then resting the head back on the neck again.
If that's not the case, and you have a low temp hot glue gun, put a generous amount of hot glue around the inside bottom of the head (around the hole where the s-hook comes out) that should help grip the s-hook.