Hold a treat in front of your face, and say the command. Offer your cheek towards your dog, and wait until he touches it with his nose.
As soon as he makes contact, pull away, and give him the treat before he has a chance to lick you.
If you are fast, your dog will eventually learn that all he has to do to get a reward is to lightly press his nose to your cheek.
If your dog likes to nip when overexcited, do not allow children to perform this trick with him. It is also wise to take care with extra-large breeds as a small kiss can easily turn into a painful head-butt. If your dog knows how to be gentle, you can take the trick to the next level.
Once he understands the command, teach him to give kisses to other family members and other dogs. Start by pointing at the other person and saying the command. You might have to give the person a treat to reward your dog with the first few times until he fully understands.