a. What happened with him was that he had an accident.
b. What happened to him was that he had an accident.
Is there a difference in the meanings of these sentences?
I find (b) more natural, but it seems to me that sentences like (a) are used and changing the preposition does not really change the meaning much.
There is often a big difference between written language and spoken language. Speech is usually extemperaneous. We don't always know how we are going to end a sentence when we begin it. For that reason, transcripts of everyday speech would not always hold up to grammatical scrutiny.
From a written language perspective, both your example (a) and (b) are unnecessarily long. Ideally, one would just say:
He had an accident.
At face value, I'd say that your example (b) sounds better - an accident is something that happens to somebody, not 'with' them.
"With him" is possible, but the meaning is different from "to him".
E.g.
you see a guy with bruises all over his face. You ask, "What happened to him?"
It suggests something bad: he was in a fight, a car crash, some sort of accident.
or
"What's happened to Obama?"
This suggests that things aren't going as well for President Obama as they used to.
A girl might say to a friend, "You used to love him so much, and now you don't even want to see him. What happened with him?"
The idea is, what happened when he was present? What was going on between the two of you? That is, there was something going on that effected both of them and their relationship. What was that?
"Happen with" is also often used with objects. A boss might say to one of his staff:
"What's happening with that report I assigned you?"
That is, what are you doing with this report? Are you making any progress on it?
The second one you suggested sounds better though in this context.
Check out this
"What happened to him?" is a purely neutral way of asking, literally, what happened to him.
"What happened with him?" is similar, but has an added connotation of dismissiveness and ridicule, suggesting that the object of the enquiry is making a big fuss over very little. This question may well be asked in a mocking and derisive tone of voice.
The scolding reply would be:
The original speaker would then be shamed into apologising for making the assumption that the subject is either fishing for sympathy or swinging the lead, or whatever metaphors you may choose for making more of an adverse situation than is merited.
