Well, to be honest the loss of his father probably is playing a very large role in his mood and lack of success in school.
As a social worker, I think his behaviours are fairly typical of an 18 year old. I can say most 18 year olds probably are not home much at all. This is an age of exploration and boundry testing. Trust me, I work with 9 youth between the ages or 16 and 22.
Do you know what he does when he is out? Is he honest with you?
I think it is positive that he is coming home, but the best thing you can do is let him know you are supportive of him, that you understand that he is going through a rough time in his life, thatyou love him, and that you want the best for him. Focus on his positives and ask him how he could improve and what his goals are. I don't think it is unreasonable for an 18 year old to be out late some nights or have overnight s with friends, but maybe set your boundries a bit more firmly. Ask him what is reasonable and what he can commit to AND HOLD HIM ACCOUNTABLE if he doesn't follow through.
Looking at the situation for a lense that sees stregths will do wonders for you. He probably feels pretty down, but has difficulty expressing his emotions.
I hope this helps. If you need anything else, let me know! Youth are my specialty!
I feel for you