Oh absolutely you need to complain, poor chap. The cut is so obviously deep enough to have required medical attention. You definitely should have been informed earlier. I teach this age group and, if ever we're in doubt regarding the severity of an injury, we would always ring the parent and then it's their call whether to collect early to visit A&E. Better to be safe than sorry.
Added to that, any facial injury can be alarming for parents to see upon collection if they haven't been pre-warned. Even superficial ones. We would always ring to warn parents if their child is likely to have a black eye or has any sort of graze on their face, even if they are fine, just so parents know what to expect and don't get a nasty shock at home time.
I would put your complaint in writing (including the photo) and hand it personally to the headteacher, asking how they intend to address this error to ensure it doesn't happen again. At the very least it sounds like the hook needs removing, as the next child might not be so lucky. Bear in mind that the headteacher may not even be aware of the incident though, so don't go in all guns blazing. I would also speak to the teacher, to inform them of the actual severity of the injury and the fact it required stitches. Reiterate that you understand it was an accident and do not blame them for what happened, but that you are concerned you were not informed and your child was left that long with a deep, open wound, risking infection. As he's in Reception, he could have easily flicked sand or something into the wound once sent back off to play.
Hopefully you get a sincere apology and they will learn from this. I would be mortified to learn that a child required stitches on an injury we didn't even inform parents about until home time. They are only human, but they need to realise how serious this was to ensure it never happens again.