if your kid can smell cigarette smoke in the air, it could be harming kid health. Dont worry, be careful...
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.