Hi Olivia,

Yes, his head is quite heavy indeed. Adding extra fiberfill would be the best advice.
If your project has the head sewn onto the neck, you will want to add extra stuffing as you make the final seam. Make sure the top of your body is firmly stuffed before you start sewing the head onto the body. Once you are halfway done add a little more stuffing being sure to shape the shoulder area as you'd like. Just before closing, continue to add stuffing until you feel it is full. Using the end of your crochet hook can help.

but here's some other tips, if that doesn't work:
- You could try sewing around the neckline twice, the extra stitches give some added strength.
- You can add a folded pipe cleaner (or even regular wire) to the neck, pushing the ends through the head where you'll attach it. If you want to be extra safe, fold and wrap the ends with yarn or tape.

I hope this helps!

PS: stuffing info was found here.