The truth is, he's responsible for this, or should I say his lack of responsibility. Suicide is not laughing matter, jokes or not. You should be a support to him, regardless if you have time or not. He's reaching out for help and if you ignore this, it might not end well. I suggest you aid him on the road to responsibility. That means he has to stop his addiction to video games and he has to focus. It's not going to be easy but if he succeeds it will change his life. I think you need to bring your mother into this, but it really depends on the relationship he has with her. I cannot stress this: what he needs is support. Let him know that right now he shouldn't worry about a social life or others, he needs to focus on himself. Once he deals with that, then he can go forward with those other things.
If the job he has right now isn't cutting it, have him find another job. Either that, or he needs to improve relations with his boss and mother. Although he is failing the course, he needs to remember why he's been dreaming of working for the police. In essence, he needs to reawaken his original motivation for this career path. If he doesn't want to do it anymore, he has to do some major soul searching. Spend time with him and help him control his spending. I think he needs to stay away from those friends because they are not worth it and they are contributing to his depression.
If all else fails, seek professional help.