My 6 year old son got a root canal

My husband took my son to the dentist. I thought it will be a simple large white filling. but he performed a root canal and a stainless stell crown on top of a white filling.
what can I do to prevent my son health problems?? (its a left top 2nd molar)

I am very upset that my husband didn't communicate with me. if I knew the dentist wanted to do a root canal, I would demand my son to be taken to a biological dentist for a second opinion.
is it too late to save my son's health?? will it damage the future tooth that will come out in early teen years??