If you can do some nappy free time and let some air to it. Keep it clean, wipe gently and keep dry. It should heal relatively quick. Also if he’s in a lot of pain you can give Paracetamol especially before a bath or shower - He will likely cry and fuss a bit when the water gets to it. Try getting some foods that aren’t too acidic and will help soften poop up and to avoid further strain. Has he been unwell at all recently? Or eaten any specifically acidic food? It looks split from irritation and looks too high up to split from straining
Let it breathe firstly. Also, has he been straining or had hard poos? Looks like a cut due to a big poo. Or he's somehow scratched himself? He is showing signs of discomfort? If not, then I wouldn't worry too much, it'll heal itself , but do try to keep it dry as much as you can to allow it to heal. If he's in discomfort, contact your drs for some cream/remedies to help