Have you had a good look at his legs and tried moving them about a bit, both in the knee and hip, to see how he reacts. Also feel all over legs, and the rest of him come to that, to see if something's digging in him.
Same with his paws. Have a good feel of his paws, in between his pads as well.
I've got a 7 year old cavvie mix and he was just curled up next to me and I was stroking him and I pet his leg and he wimpered even though I didn't touch him hard at all it was just my normal touch a bit like when you tap a baby on the back when they are crying and you're trying to sooth them so harder than a stroke but not a hit obviously lol.
Can anyone suggest why he would wimper when I did that?