Yes, of course share your concerns with him.
But here's the thing, college is a personal choice. There is no LAW that says he has to go, and furthermore since is is technically an adult, there is nothing at this point you can 'make' him do.
This is where you sit back, take a deep breath, hope and pray you've instilled enough self respect and desire to excel at life, support him, cheer him on, and CUT THE STRING!
I mean, if he gets a bad grade, what are you gonna do, ground him? Ridiculous!
(I say this from the heart, I have an 18 yo who went of to college in September. He his holding his own, having a blast, experiencing new things, keeping his 3.83 GPA alive. He was not a troublesome teenager, in fact he never once while in HS went out with friends who were driving, tried a beer, smoked, nothing. I realize he is an unusual teenager, I also realize he's trying it NOW, while I'm not there. But he WANTS to be successful, he LIKES being a brain.....I should add, he after his scholarships Pell Grants and a little help from his Dad, he will be paying for much of his own education.)
There is no contract, and it's very SCARY relinquishing that moment to moment control you think you had in his life while under your roof, but it's ultimately up to HIM, not you, what he gets out of college.
:)