I am going to take what is probably an unpopular position...
If your boyfriend wants to be a grown up and live life the way he wants to and not have to answer to his parents, then he needs to grow up and take responsibility for his life.
That means, he needs to quit eating their food, quit sleeping under their roof, take himself off their insurance plan(s), get off their cellular plan, and get someone else to co-sign his loans. Because reality is, whoever pays the bills, makes the rules. If he doesn't like their "rules", then he needs to move out from under them. And to sneak around doing what he wants while still accepting all these things from them is a lot like cheating, in my book.
We may disagree with his parents views or how they've reacted, but as long as they are supporting him, they have a stake hold in how he lives his life.
My question for you is, are you willing to replace the support his parents are giving him should they take it away? If not, how loving is it to encourage him to risk it?
Me: 48 - Married, straight, male
Shiela: My wife.
John: Her boyfriend.