I overfed my 9-week Westie puppy. Will it impact his growth or joints?

As long as he is active and comfortable and able to pass stools then any mild food bloat signs should pass, a mild laxative such as cat hairball paste or liquid parafin can help. I would contact the breeder to check the brand and feeding guidelines, or transition to an alternative puppy food by mixing 25% of new food guidelines with the original diet, increasing proportion every 3-4days until change is complete. No permanent changes in joints etc would be expected after a few days of overindulgence (avoiding puppy obesity and maintaining a balanced, puppy diet with appropriate mineral and protein levels is important in the medium-long term however)