It is highly unlikely a cigarette on the ground would transmit an infectious disease to your son. Most viruses do not live very long once outside of the human body, though it's always a good idea to wash hands after playing outside.
This may be a good opportunity to start educating your 2 year old on the dangers of cigarette smoking. Teach him not to ever pick them up, especially when he’s older and others around him may be smoking.
if your kid can smell cigarette smoke in the air, it could be harming kid health. Dont worry, be careful...