Everybody has opinons,some good,some questinable.
Between the time I had my two children, my pediatrician's opinion changed. She went from being a strong supporter of children being at home with Mom (which I wanted to be but wasn't) to children being in some sort of organized child care consistently, even if only 2 days a week etc, part time, whatever, because of the socialization that was happening. She was seeing children at 4 in prek having a terrible time because all the other kids had been in day care and had learned to follow directions, stand in line, and do all that nasty stuff kids have to learn. That being said, I remmeber having my mother in law care for my older child, and he did fine after about 2 months settling in when he started prek. But standing in line was really hard for him.
Look for where your child is. If he is happy, enjoys the company of children, socializes well, and the school,the teachers, aides etc are all above board, celebrate his going to school,make a book, go buy an ice cream that only boys who go to school get...the baby doesn't get one cuz he isn't in school, all that good stuff. If you have any question of what is happening to your son at school, find a way to check in and monitor etc and make sure there is no bullying and nothing adverse from the staff. My older son went through this too, at an older age, and his issue was was with a teacher who wasn't teaching and being mean, and some bullying from other kids in the class. After working with the school, we pulled him mid year. I didn't feel to badly,by the end of that school year the school had lost 1 full class room of children out of 3 class fulls that started in the fall. The school had a lot of problems that they were not fixing (this was in grade school). So, more words to read. Good luck. Remember, everybody is doing their best, and your love and support to your children,whether in child care, preschool, nanny school, commune whatever, your love and involvement is what is going to make the difference to your child. Let them be children, let them play and get exercise, let them socialize, and love them. Good luck.