I currently am in Engineering freshman too at Northeastern University, and I can understand where he is coming from. I was itching to get out of my house, so I could be on my own and not have my parents all over me. I was a good student in high school, 28 of 466, although I think my final rank improved a couple.
I can't stand work, I get frustrated a lot and just don't want to do the work. I find that I just don't have the drive to my work, like I get back to my dorm and just don't want to watch tv, play games, hang out with friends, or just do nothing, come on my computer.
I can understand where he is coming from, I have felt tons of pressure and anxiety at times and just didn't think I could get through things. I was extremely happy at the beginning of the semester because I was in a new group of friends, I was doing pretty good, the scenery was different. Then I got mono (which didn't really affect much except I never went out because of it), the people I was friends with would started to hang out more with themselves than with me, etc. And I couldn't wait to get home for Christmas, and see all my old friends. So time comes and I get home, then all my old friends don't call me up, and now I'm back up here, and things are good again, I'm hoping the semester will stay like this.
Tell him to do his work, make sure he goes to the library or something, that is where I'll actually be motivated some to do work, and also if I get into a group with other people I'll try to study more etc.
And I have thought about this myself, maybe college just isn't his thing. I have doubts about college myself, and I actually enjoy doing manual labor like building things etc rather than trying to do brain problems and all of this most of the time.
I think the best you can do would be supportive of him, and tell him that your always there for him and you are proud of everything he has done so far. Also that you respect his decisions, and you might have to think that if college really isn't his thing, then stayin there might be hell for him. Tell him things could get better and for him to try to stick it out for a while, but if in the end its not do able, you have his back. I suggest this because my parents have done the same for me and told me this and it releives some stress knowing that I'm in college for myself, and I'm not going because my parents have expectations or its required that I go because they think its best.