Older dogs frequently greet their owner's like this, as younger dogs and puppies tend to be exuberant and bounce around the place.
The stretching, is most likely, caused be the fact he's being lying down and needs to stretch (as mentioned in the comments). It's a sign of relaxed happiness to greet you like this, as a dog generally won't do this when a stranger turns up.
The fact that the dog has his head lowered (apart from the fact it's a nice stretch) and not the opposite (with his behind lower and head stretched up) is an indicator that the greeting is non aggressive.
If your dog is submissive, he’ll lower his body or even cower on the ground. His head might be raised, though, if he’s greeting a person or another animal.
From Canine Body Language ASPCA.org
Chances are he gets a pat when he greets you like this, so he would have learnt that this is a smart way to greet his owner, as it i not boisterous, nor domineering, but endearing and he gets a pat.