Not ready to send our son to school

I'm the breadwinner in our family. I run my own construction sight. My husband has been a stay at home dad since our child was born. Before that he was a house husband. He's been kind of depressed lately and told me he wants to go back to school and if we can send our son to school this year so he can do so. Our son will be 4 so he can go to pre-k. Pre-k isn't mandatory. At least not in our state and tbh I'm not ready for him to go to school het. He's such a sweet kid and I'm worried about the influence other kids will have on him. My husband hasn't had a job in 6 years so he's wanting to go back to school for a teaching degree. He was going to school for teaching but dropped out years ago to take care of me when I had cancer. After I beat cancer I went back to work and at the time from being out of work we didn't have money to send him back so he just took care of the house and I earned the money.... I know how bad he wants to go back to school. I just don't think I'm ready for my son to go.