I am going to talk in generalities here because I am not as familiar with voice as I am with instrumental music. If your son’s grades and test scores are not great that won’t necessarily rule him out at a music school inside a university, but it can make it harder to get in because he needs to get accepted academically and in the music school. They give leeway to music students with the academic side, but if the grades are bad enough it could keep him out of some of the programs (not sure about Oberlin, I thought you could apply only to the conservatory there).
As far as scholarships go, it depends on the school. With conservatories, they generally tie both financial and merit scholarships to family need, unless someone is that outstanding and even there there are limits. In the end it is going to depend on how much the school wants the student, if they are really, really good and are trying to attract top notch students (more at the programs trying to build their reputation then the big, established schools who attract a lot of top rate talent). From reputation (and take that FWIW) a school like UNT might be more generous with aid than a school like Indiana or CIM, especially if the kid is good (that doesnt’ mean Indiana or CIM would not give your son a great package, I am talking relative tendencies).
With VP, it also seems to depend on the type of voice (and I home Mezzosmom and some of the other experienced voice parents chime in on this, they know a lot more and will be better with specifics) , sopranos from what I have read over the years are a lot harder to get merit aid then someone with a rarer kind of voice, in a male voice I don’t know how rare a basso profundo is but if it is relatively rare it may get him more aid.
On a more direct note, have you looked at the admissions requirements for the various schools and seen what they will be required to audition with and has your son gotten to that level? I don’t know if voice programs require pre screen submissions, but those generally are due by early December, and if he doesn’t have the required audition pieces done by now it might be a stretch to make that…and auditions are usually in February and March roughly, so if he has big gaps in his needed audition stuff, it may be a tall order to get to the level needed by audition time . Hopefully your son has been talking to his teacher about it and putting together what he needs, last minute prep usually does not work as well as people might thing.
One note, if your son wants to teach voice, he only would need to do a music ed degree if he planned to teach voice in the public schools, teaching privately or in a university (you mentioned professor) does not require a music ed degree and quite honestly, If he has any hopes of performing or teaching at a high level I am not sure the music ed degree, which requires a lot of things outside the direct study (in his case voice) would be good prep for that, and unless he wants to teach in the public schools a music ed degree would not be the way to go.