N., I hate to sound negative or overly worried, but I see red flags flying everywhere! Something has happened or is happening to your child during school. Have you talked to him about inappropriate touching from others? I have daughters and I've always talked to them about how NO ONE but mommy and the doctor, and the doctor only when mom is there, are allowed to look at their private areas or touch them, and only to bathe or see for health reasons. With my son, I've been very worried. Boys just don't really get it because showing it off seems funny. He's only 4 but we still talk about it. When he had a urinary problem, I had to explain to him why the doctor needed to see his "weed-a-wacker", don't ask, my aunt names it this and teaches all the boys...He was fine, as long as mommy was there.
Again I don't want to scare you but something is going on. Probably not to the extreme of sexual, but another child teasing him or hitting him, or something, but obviously your child has something going on. Yes, they can get moments where they just want mommy, esp with you being a single parent. I'm a military wife so basically a single mom with an extra paycheck...which helps but still doing it alone. If he was fine when you picked him up, I would look at him just wanting to be with you. If he's still "out of it" at that time....it has to be something else. You need to talk to him and find out what happens at school that makes him unhappy. It may be a little tough, but keep talking. Is he ever the last child picked up? I would say do the surprise arrival idea, but with having to return to work, I know that never worked for me. They thought if I had arrived it was time to go home. Hysteria all over again. I wish you the very best and hope it is nothing serious at all. Our babies are our lives. May even be his best friend left...I dont' know. I know my son tells me everything that happens at school, but at 3 I had to ask alot. Good luck and God Bless