Our son has taken over the Sunday tea. It is terrible and we drink every drop

Since class 11 began, our 15 year old has made the Sunday morning tea for the household. He measures nothing. Last week it was mostly milk. This week it could have stripped paint. My partner and I drink it on the balcony and tell him it is improving. It is not, but he has started asking how his grandmother made hers, and I am not going to be the one who ends that.