If he's not neutered, he should have no problem. Try rubbing gently at his penis using his sheath; don't touch it directly as it will hurt him unless your hand is very soft and slick with lube. Rub around the tip and kind of massage backward toward the base, pulling back the sheath as you go. That should get a reaction.
Due to his small size, it might be a good idea to approach him from his front with your knee right by his head. I've had lots of luck getting small dogs (and large dog puppies) to mount my leg when I masturbate them. They enjoy it a lot more when they have something to cling to and hump at.
If it works, he'll start thrusting and making a godawful mess squirting all over (never underestimate how much a small dog can cum) so I hope you remembered to grab a towel. Just squeeze gently behind his knot until he finishes humping, which means he's done having his orgasm. Let him go and he'll go lick at his dick until it goes back inside his sheath.