When do you say this? How do you say it another way?

I let him have it.

Question

When do you say this? How do you say it another way?

I let him have it.

Answer

Just to let you know, "Let him have it." is also an idiom, meaning to get angry, etc. at a person.      EX;

"I told him to stop bothering me, or I was gonna let him have it."

This usage may be becoming slightly antiquated, but it's still in use.

Answer

It can mean different things depending on the context. Please give context in a story.

Answer

He asked you for something and you agreed to give it to him.  The thing in question could be anything. 
'He asked if he could use the computer at 5pm.'
'I let him have it.'

'She wanted the time off next Wednesday to see her son.'
'I let her have it.'

Another, more formal, way to say this would be
'I agreed to /accepted his/her request.'

ご自宅で快適に語学を学べるチャンスをお見逃しなく。経験豊富な語学講師陣の中からお選びいただき、今すぐ最初のレッスンにお申し込みください！