As a retired school-yard fighter I have some advice to offer. Some has been covered already, but my EXPERT OPINION on the matter should help bring it all togehter.
1. The other kid(s) are expecting him to be scared. First thing he should do when he gets to school tomorrow is find the other boy and say, "Where and when are we doing this? I want to make sure I'm not late."
2. When the "fight" is set to begin, the expectation is that there will be some talking and jawing and swearing followed by someone shoving someone else. If the other kid comes in for the shove, your son absolutely without fail needs to punch him in the nose. Not the chest or stomach or chin but the nose. If he lands one good shot on the nose the fight will be over.
3. The same goes in reverse, if the other kid hits your son in the nose your boy will end up on the ground crying and that will be that.
4. In the unlikely event that the fight actually turns in to a real fight and not just some kids swearing at each other, he wants to take the fight to the ground if he's bigger and stay off the ground if he is smaller.
5. Kicking the other kid in the nuts is absolutely acceptable. Biting is not.
But I'd bet good money that if he does #1 with any confidence the other kid won't even show up.