Jimbo has a slight mocking tone to it, more than “Jim” does for James for example. It is not a regular diminutive in the way Sasha would be for Alexander in Russian.

If you know him well and also know that he likes to be called Jimbo

You can if you know him well, and he's an equal - a fellow-student, for instance. I wouldn't call him Jimbo if he's your teacher or priest or boss. He might consider it disrespectful.

There is no hard and fast rule about this kind of things. Personally, I think you have to know James very well to call him Jimbo.

Yes of course