Mirilax for 13 month

Y’all. My 13 month son has hard stools and just dances and is in pain. I’ve cut down milk. More juice and water. Prunes pouches. Even did lactose free milk. Still hard. Called pediatrician again, and they suggested 1/2 cap of mirilax daily. Do they grow out of this? Is it just because he’s still adjusting to milk? Idk 🥺🥺