Weak back leg in my strong 7yr old boy should I be worried

My 7 year old very muscular boy has never been a keen walker. Don’t get me wrong… He’s keen to walk to the bistro where he’s in with a chance of food, & very keen to walk around the shops there he is admired & adored… But a general stroll around the block is a real battle of wills. He’s always had seemingly slightly wobbly back legs legs but had all the necessary tests as a pup, he is a pedigree. Recently he appears to stumble on his back leg after being asleep – almost like his leg has gone to sleep. He also developed a limp on his short walk today after a mini sprint (his choice) but immediately sprung back to normal when we got home & it was treat time. Is this something I should be worried about?