I’m pretty sure all this means (based on someone who posted UK law earlier) is that they’re giving him an interview under the assumption he committed manslaughter and is pretty standard act.
That seems to be the case but we'll find out in 24-48 hours for sure. He'll likely either be
Retained in jail (this is for serious crimes / criminals)
Bailed (which means the police believe they could have enough evidence for a conviction or have an admission of crimes)
Released under investigation (meaning the police are still investigating the full extend of the issue)
Released
The top and bottom of that list are the least likely in this scenario but are possibilities.
Just for clarity I'm not a lawyer, just know parts of the process.