Ouch, that does look a nasty cut but unfortunately accidents happen. I'm glad he's doing ok.
I honestly don't think you'll gain anything from speaking to the teacher or even the headteacher. The teacher can't have eyes in 30 different directions and if she didn't see exactly what happened, what do you expect her to say?
We had drawers in my classroom and you can tell children until the cows come home, to close their trays/don't shove your jumpers in there etc etc etc but they're young and they're in a hurry! Also, they have to be down low so that the children can access them!
Wrt the accident form, at my school it moved to all online. You filled out the child's name, a brief description of what happened and what injury was sustained, treatment given and what action came next - child stayed in school/child taken home/professional help sought etc.
I presume the school has followed this because you were called, which is what would have happened at my school with ANY head injury, even very minor ones.
I appreciate that you would have been shocked to see your little one's cut face but I really don't think the school can give you any more information I'm afraid. Unfortunately, it's just one of those things.
Oh and I'm sure the class were reminded about closing drawers again but I imagine these words went in one ear and straight out the other! They're little children and accidents happen.