If he doesn't drink it I think it will stop on its own.
"飲まなくても放っておいたらそのうち自然と止まると思うよ"
は英語で
If he doesn't drink it I think it will stop on its own.
といえます。
他
It will go away on its own.
勝手に止まるよ。
It will go away if we leave it alone.
放っておいたら、勝手に止まる。
「薬飲まなくても、放っておいたら、そのうち勝手に止まると思うよ、だから薬飲まなくてもいいんじゃない?」
Even if s/he doesn't drink medicine, it will go away on its own eventually. So isn't it fine if s/he doesn't drink medicine?