Thank you for the measurements. That removes a little bit of guess work.
The cut is small and should heal quickly as long as you've cleaned it well. If you go out tomorrow, buy some Vetericyn and spray it on the cut. Do it three times a day. It has the ability to hurry the process of new tissue growth. The cut should close on its own.
Follow the guidelines above and do not give food or water to the roo tonight. You can give him water in the morning, but no food. The wound should be closed by tomorrow night and by Tuesday, you should be able to give him soft watery foods. Nothing rough since he won't have grit in the crop to help digest it.
I would feed him yogurt, plain, soft boiled egg, and maybe baby food veggies and fruit for a couple days while the cut is healing up solidly.