I can't comment on whether or not contribution to the 529 plan is a gift to you.

It might be reasonable to open a separate plan with them or your son as the owner.

Depositing funds is the easy part. The Fidelity form at the link below should be all that is needed.
http://personal.fidelity.com/accounts/pdf/genericdepositslip...

Since the end of year is approaching, gift limits might not be a concern. Each of your parents can gift the annual limit of $13,000 to you (annual total $26,000) and if you are married to your spouse (annual total of $52,000).