G.R.
At 17, your son is nearly an adult. As you know, we can make adult decisions long before we are legally entitled to. You have to let your son make his own choice here. If his father is still as irresponsible, it is only a matter of time until your son is disappointed by him. Seeing our parents as people is a really important part of growing up.
The other thing I would urge you to do is remove your objections from the conflict. Teens will rebel against rules and by giving him something to fight against, it may take him longer to see his father's shortcomings. By forbidding it, you are pushing them together and giving them something in common--complaining about you.
In this instance, as painful as it is, you've got to let your son discover who his father is. If you manage to keep this from turning into a conflict between you and your son, he will be able to talk to you about his father when he needs to. And he is going to need you to help him.
This holds true with all sorts of problems with adult (or almost) children--an open door, nonjudgemental adult parent can be a tremendous resource. This may be what he thinks his father is, mistaking permissiveness & irresponsibility for acceptance. He will have to learn this for himself.