Loan an Apple Watch

I guess it all depends on how you set it up for him. Does he have his own iPhone? If he does then set it up to his iPhone with his Apple ID.

Set up your Apple Watch - Apple Support

Then when he is done, completely erase the watch and make sure he removes the activation lock from it:

Unpair and erase your Apple Watch - Apple Support

If he doesn't have his own iPhone, then set it up according to:

Set up an Apple Watch for a family member - Apple Support

Again, when he is done, completely erase the watch and make sure to remove the activation lock.

Finally setup the watch to your iPhone and use your Apple ID:

How to pair your Apple Watch with a new iPhone - Apple Support

You will have to unpair the watch from your phone then pair it to his,then when he's finished with it he will have to unpair from his phone then you repair it to your phone and restore it from the backup that was made when you unpaired the watch from your phone before you gave it to him.