I'd gotten in trouble with the law when I was just of age. I didn't go to prison, have not been convicted of anything more than a misdemeanor, but my criminal record has been an issue with schooling.
I know things vary depending on where you apply, but it took me about a year to transfer from a community college to a university. It's going to depend on what he was charged with and the dispositions on those charges. My impression is that, for felony charges, schools want for you to have been off probation for five years. They're more lenient, the lesser the severity of the charges. Some no-name schools won't care.
As far as the atmosphere, it depends on him. I go to a R1 school that tops lists for being a party school (it's PGR ranked and top in its specialty on PGR--I don't think, however, its undergrad philosophy program is very good) and I've never once went partying there. It's not really my thing. It'll depend a lot on aspects of his character, considerations of the school's environment. It's a little presumptuous to assume that because he's been in legal trouble what would or wouldn't be a good fit.
Not knowing what kind of program he's in--if he's earned an associates' or if the program sets people up to be accepted to colleges--assuming this is in the US, I would expect that his best option would to be to go to a community college for a year or two and transfer to a university.
If I were giving him advice, I'd tell him go to community college then shoot as high as possible with transfer applications. I didn't apply to many places because I didn't think I had a chance of being accepted. Once I did get acceptances, I regretted that I didn't apply to better schools. With undergrad, I think the pedigree of the school overall would be the biggest consideration.
It might be good for him to get away from wherever he was and get a fresh start. Old friends and being thrown into the same environment he was in could be a problem.
Depending on his age, you might try looking into schools and programs for non-traditional students. UPenn and Colombia have them, though I don't know that they have philosophy programs. They also accept transfers.
The stigma of having a criminal record socially at college is not that big of a problem. Again, if you keep it to yourself, most people won't suspect it. If he has mugshots online, getting or trying to get them removed would be good advice. People might look him up and find them, but otherwise, if he doesn't bring it up, no one will likely suspect it--unless he has face-tats or acts like a goon. If he's a bit older, people are more likely assume he did military service than that he was in prison.
Sorry if this is not relevant. I know you said to bracket practical concerns.