“It does not mean that you must tell him a lot of jokes, though it may be one way of keeping a person in good humour.
The word ‘humour’ is most often used in the sense of ‘capacity to cause or feel amusement’.

He has a fine sense of humour, but she has a delightful sense of humour.

‘Humour’ has other meanings also. One of them is ‘state of mind, mood, inclination, temper.’ Good humour means ‘cheerful mood, happy state of mind’.
‘You must keep him in good humour’ means that you must keep him cheerful.
For eg:
When I went to see him, he was in good humour.
He took the joke against him in good humour.
When one is moody, you say that the person is out of humour.
Don’t go to him now. He is out of humour.

‘Out of humour’ means ‘displeased, vexed, out of sorts, out of mood.