You've got to be open to the possibility that whatever you say he can still resist your advise.
It would be best to focus on his goals (what he wants to achieve) and point out to him that he would need to stay focused on those more than anything else. He may be trying to fit in with this crowd of "friends" and you'd want to see if they have similar goals/aspirations like he does. If they are not doing well with their school work, then you let him see that he may jeopardize his own future.
With him being away from home he believes that he had found a whole new freedom now. If you are responsible for paying for some of his expenses, then you may ask him to get a school study job instead of hang around these friends. He has to chose what to give up - his friends, studies or give up some of his free time that he uses to hang around with them and work/do his schoolwork.
At times words are not enough to convey a message. You may resort to actions if words do not work 100%, and he will have to unfortunately in some cases learn from experience if he resists what you tell him.
Thanks for the answer I will accept in a little....my daughter is away in college too she does well in most areas of her life she is a 17 year old but seems to be handling away from home very well. Her issue is she is very social and do not know how to immediately move away from other student who is not focusing on their studies. As I said she does very well in her studies but very often she has to step away and do some damage control because she did not immediatley act on intution...she ask me for advice on how to make appropriate without hurting anyone feeling. It seem as if because she is social she wants to protect her freindship with eveyone but it could be to her detriment.. whent are the points that I should focus on in order to develop that self-confidence in her and keeps her motivate to contnue do well and make smart choices.
You may want to compliment her work (achievements/accomplishments) most and for all. That can act as a motivator for her to continue to do well and excel. She is still young and is learning about how she fits in at college and with her peers. This is a testing/learning time for both of them. You would want her to trust you and to respect your opinion. There will be times where she would make her own choices (good or bad) and she may want to know that if she's to make a mistake (everyone does), that you would still be there for her.
As far as hurting other's feelings (people chose what to feel- the other one is not fully responsible for that) Some flexibility is always helpful in trying to approach others and their feelings. What comes out in the end, is their choice though.