Armor? Are you taking him on A-Line or flowy singletrack?
Aside from a decent helmet and good gloves, I like the Alpinestars Paragon kneepads. Slip-on, and after a few minutes you forget they’re there. Don’t slip around either. For hips, I have a pair of Fox chamois liners with padding on the sides and tailbone. Forget what model they are, but I’m sure you can look them up (I’m too lazy to search right now). They’re really comfortable, too.
Taking your dad for a rip on the trails is about the coolest thing ever. You earn cred just for posting this!