See reply to "children and cursing" below. That may help some. Also, you can search this site for cursing, and see what that brings up.
Best wishes.
Dear Tony R.,
Your grandson will stop using the foul language when he experiences that it is not in his vested interest to use such language. If you check the questions from March 22, you will see my reply to this very circumstance, and I'd reiterate it to you. It'll work, and you'll be helping your grandson to behave in a courteous, civil manner.