I would not get involved. If he's not driving to your school under some kind of influence, he'll only do it somewhere else.
And I know you think life would be easier if you didn't work with him, and this may be true. But it sounds to me like,, if you're patient, he'll get caught eventually anyhow, when those behaviors become more pronounced (which they will if he keeps up with it long enough).
The other thing is I notice you say you "think" he's getting high or coming down off of something. Are there other possibilities here? What if you're wrong, and you tell on him, and then you end up looking bad?
This is all just my opinion, of course. Just be careful, because us codies can relapse too.