R.J.
Yup. Absolutely a medical power of attorney (print, take to the bank sign & notarize) because even lifesaving surgery will be postponed to the very last moment if they can't reach you... but for the more likely non-life threatening stuff... you don't want your son sitting in the ER with a broken arm and no pain meds for hours because you can't be reached (seen that happen).
ALSO... phone your pediatrican's office and let them know that your parents will be caring for your child for x number of days and ask if there is a consent form of any kind that they would want you to sign.
Ditto, a handwritten note (with a photocopy of your drivers license and your phone numbers) authorizing your parents to be taking care of your child and traveling out of state with him has as little chance of being needed, but if for any reason there is an amber alert, or they're pulled over, or, or, or... cops want something that verifies that they are allowed to be traveling and caring for your child.