Why dont you pay him any rent? Houses are expensive and you are using electricity, water, gas etc. Im sure he would appreciate it if you just decided to pay your way.
Edit: Since im being downvoted i want to add some context.
The tone of the message im responding to (or at least the way it read to me) was that this person is taking advantage of their dad. They said
Why wouldn't I take the deal to have no rent while I work full time at a decent job so I can pay off my student loans? Sounds stupid to move out at least for me right now
It doesn't come off like they have discussed and agreed on anything with their dad.
It sounds like they are earning a decent wage and haven't even considered getting their own place.
It sounds like they want to milk their dads generosity for everything they can so they can have an easier life.
Now ABSOLUTELY i cant be sure and could be completely wrong. But that's how it reads. Like they don't give a shit and are acting selfishly and dont even see how its selfish.
To be clear, i dont think the dad should impose rent or any kind of fee on them. I know how hard it is to get a home as a young person, and i likely won't be asking my kids for rent. I just found the attitude i perceived in that post to be asinine.