He needs a Ninja Knot! (trademarked by Nancy1999
). It's called a topknot.
If the hair is long enough to get into a little band, go for it. Make sure it isn't too tight. It should not pull the corners of the eyes up, and it should not pull the skin, otherwise it could create a sore or worse. Be careful with the ear hair too. Don't get that caught up in the band.
It may take lots of practice to get him to accept it.