
Could be his joints are causing pain but would think it may be soreness or irritation between the pads. Some dogs do get into the habit of doing things like this for no obvious reason, at least no reason we and the vet can fathom, would guess the dog knows why.
I have a dog of 7years who has always put one of his back feet in his mouth, it is now a pink colour but we have no idea why he does it, bit like a child sucking a thumb it is with out reason but it still happens.
Would inspect the feet between the pads and see if you can see a reason and ask the vet when next there, or sooner if you are worried. Could be something he has walked through that has been sprayed on the grass or a change in your cleaning materials.