Does "He ended up in prison." make sense? I think "He ended up going to prison" makes sense. Which is more reasonable? The former seems to lack the middle process.
ex)He got involved in a fight at the party. He ended up in prison.
He ended up in prison is fine. I much prefer it to your version.
end up being in prison = end up in prison
Draw a timeline. Mark the end state as 'in prison'. Your 'going to prison' is accurate in describing what he did, but when you say 'ended up' the listener expects to hear about the final state.
b
I've had a further thought. The timeline idea is OK, but ti all depends what you put at the end of the line. 'In prison' is a state - so 'he ended up in prison'.
In some cases though the end of the timeline is not so much a state as a process: 'First he ate one piece of cake, then another piece to make sure the two cuts met at the exact middle, then another piece just to tidy up the cut.... He ended up eating the whole cake'.
Of course, it's usually (always?) possible to argue that a process ends in a state (and vice versa: the state of 'being in prison' is the culmination of a process (being arrested and tried, found guilty, and going to prison; and the process of 'eating the whole cake' ends in a state (satiety/bloatedness... or just 'having eaten the whole cake'). So whether you choose to focus on the state or the process in your choice of verbs and verb forms is up to you.
b