Talk to your son's father. Simply tell him that if the GF continues to smoke when your DS is around his house, then DS will no longer go to his house. Simple!!

Your ex will either need to make other arrangements (i.e. going shopping, visiting your place, going to a park, staying at DS's grandparents etc) to spend time with your son. Or he could tell the GF to stay away while your DS is there (assuming they aren't living together full-time).

Ask him if the GF is more important than your DS.

As you can probably guess, I tend to be confrontational when it is about my kids. I don't argue or get aggressive, just state my position and ask what they intend to do to rectify the situation.