Do decayed milk teeth affect the erupting permanent teeth?

Hello doctor,

A few days back, I had an appointment with a dentist for my root canal treatment. I took my little son with me as he was alone at home as the maid took off. He was holding a straw and playing, and accidentally, the dentist just saw his mouth when he opened it. The dentist told me my son had many cavities to fill, but my son did not cooperate. So I brought him home without any treatment. But now I am worried about my son though they are milk teeth. Initially, I was too careless and ignorant that they were just milk teeth, but the dentist told me it could affect the upcoming permanent teeth. Is that true? If so, how do I get his teeth treated? He is not co-operative, and the dentist I met was not a pedodontist, and I prefer a pedodontist’s opinion. Kindly suggest.