If he is asymptotic then nothing is required for his age! It can trouble him on two ways in future either get infected or stone trying to slip into the duct! Many a times they live whole life without any problems, But once it gives trouble with fever and sever pain then it needs to be removed. No drugs work in dissolving it! You need only periodic check up now! No drugs, but avoid too much of fatty and oily food!

If no problem wait and watch. No magic diet or medicine  available  to dissolve. If problem because  of gall stones plan operation  later

There is no medicines to dissolve gall bladder stone