In over his head

I would appreciate some help for the meaning of the following sentence.
This is about a defense lawyer fighting the plaintiff's one in a courtroom:
"Belk couldn't be over thirty. Maybe five years max out of a middle-range law school and in over his head going up against Chandler"
Thanks for your help.

'in over his head' is an expression which indicates someone is in a situation beyond their capability to handle. It suggests swimming, where someone who can't swim gets into deep water and drowns as a result.