It is never too late to make a positive change. However, it would have to be a joint effort from both parents. I believe all children are good and will only emulate the behaviour of the role model they look up too. I assume the father, is not aware of the bad habit for the use of foul language, thus it is critical that you draw his attention to it. If the father starts implementing, a no swearing approach first (leading by example) and thereafter you both address the matter with son, you could tremendous results.
In conclusion, I wish you well in your attempt to make a positive change. With lots of patience and perseverance I am sure you will succeed.