In the US, you have nothing to worry about legally from him seeing track-marks, or knowing you use/have used drugs.
Beyond the legal matter, different doctors have different personal feelings about drug use. Some might not say or do anything beyond asking you about it, but its also common to see it noted in charts. Some doctors will also mark you as a potential drug seeker if they see outward evidence of drug use.
This can have implications for your insurance provider, who has full access to your charts. If you admit to using illegal drugs, insurance companies can use this to deny coverage in the future. A note about suspecting drug use, though, would not put you at risk of losing coverage.
Your best bet if your doctor mentions them, is to frankly ask him if you can have a discussion without him noting it in your chart. Some docs are actually concerned with the patient's health and will be cool about it, others might refuse.