I am not sure if I totally understand or not. Are you asking if it is ok to do or if you can get away with it?
You might be able to get away with it and I am not the moral police, but in my mind, it is not ok to do unless you would actually be paying for an extra room for him to stay with you that you would not be paying for otherwise and in that case, it would make zero financial sense to do it anyway. If you would not be paying for an extra room whether or not he was staying with you, then you are really not paying much more for him to stay with you or not, I think.
If you were paying for the
DDP, that would be one thing, but the fact that you are getting it for free (minus the rack rate you would be paying for the room either way), I feel like it is pretty dishonest. In my opinion.
Part of the reason the DDP has been more and more restricted in recent years is because people chose to abuse the plan and find the loopholes to do such. This would be just another example of that, IMO.
The fact that you could remove him from your reservation today, with no penalty, and even if you had to wait until after your 45 days, it would only cost you $50 and you seem to have some inkling that doing this might not be the right thing to do (thus the disclaimer at the end of your original message) should point you in the right direction.
Of course, people will tell you, "Why not?", "Disney makes a ton of money off of us, so what if we get one over on them?"...Not getting caught does not make it right.