My Bayley did that to one foot, the top of her paw was hairless, raw and bleeding because of her licking...I worried and checked the internet resources...I was afraid that she was getting a compulsive disorder (yes, they do get them.)
But on closer inspection (after checking UNDER her paw many times) I looked between her toes on the TOP and she had a small dirt clot stuck in betweeen her toes...not a big one...but bad enough that it bothered her.
Now, granted, your pup can't have a dirt clot on his leg...but when you said he had little bumps there, I suspect that something is going on with that one leg. (Perhaps a small thorn, sticker or sliver?) Maybe you could get a magnifying glass and take a look...just a thought.
I'd suggest checking into a canine dermatology...maybe it is something simple. I hope so.