have to agree with the tongue flick dog wasn't comfortable. also dog looks to be up there in age and seems to get uncomfortable when touching the rump and tail set area "by back near tail" I would always go to vet if unsure of anythingWhen you first started, he licked his lips, a Calming Signal indicating that he might not like what was about to happen. A Ridgeback has very quick reflexes, so I imagine that he didn't react, other than moaning, because it was you. I didn't see a stretch, yawn, lick, or tail wag that might indicate enjoyment.
Also, the way that he was lying down looked a bit awkward, like he was trying to get comfortable/relief but not able. A vet may discover some arthritis and provide some pain meds.
How old is he?