My dog is ok with other dogs until we pay attention or stoke the other dog then he snaps what do we do to stop?

My dog ( 6 month old american er spaniel ) ( who we had when he was 16 weeks old ) is ok with other dogs ( abit dominant ) until we pay attention or stoke the other dog then he snaps at the other dog we think he is defending us and being over protective what do we do to help stop this behavior ?