There is a friendly low growl but it's accompanied by a wagging tail. You'll see it with dogs who are well-known to each other and friends, when they meet (here, for example, adult litter sisters do it)
Your fellow obviously isn't about to attack you nor is he seriously threatening. What he's doing 'is trying it on' '. He'd try it with other dogs in the house too. He wants to see what happens if he tries it. If you appear at all compliant, submissive or don't answer him back he thinks that's a point to him. Next time he'll try something a bit bolder and unless checked this will get out of hand . Now, were you another dog you'd either go along with it, and he'd become dominant, or you'd respond in like manner. You'd growl louder and adopt a threatening posture, standing taller, try to dominate, get him to back off, fixing your gaze on him. What we are getting you to do is play the dominant dog by giving signals which he understands in 'dog language.
A dog that has you in what he sees as a submissive posture, on the floor, looking up, is quite likely to **** his leg.It's a marking territory gesture, showing dominance over it. There's an old Irish story of a great wolfhound who overpowered a man, and then arrogantly peed on him.