Both my daughters started school when they were 5, and both had really wanted to go and we didn't have any crying problems. My second oldest even didn't want to go eventually because she was a little shy, but she also said she was bored, but that is because her knowledge is not Kindergarten level. She is more intelligent and has been doing first grade work. We put her in kindergarten tho to have children her age and for the social atmosphere. She may be really smart, but that doesn't mean she still isn't a kid.
I have heard that most boys aren't ready as soon as many girls are for school and that waiting really helps their performance. I know many moms who have held their sons back one year longer from school than they do their girls just because the boys don't seem to do as well unless put in school a year later. If they didn't hold them back in the beginning, they eventually would be held back later on for academic reasons.
My children won't be sent to preschool, but it helps that I am married to a teacher and they love to have their own workbooks that they use here at home.
That is terrible that your son had a bad cut on his head. Although boys will be boys, there is no reason why he should be ending up with something like this at his age. I hope he is ok and that you can resolve this with his teacher.