You've already accepted his right to house rule the system when it is in your favor, you can't now back out and force him to adhere to the rules as written.
This is a very important thing that I wish all players kept in mind. A big pet peeve of mine as a DM is when players are very vocal when the DM implements something in the game that might be a penalty for PCs...but when the DM implements things that might actually be overpowered for a PC, nobody says a word.
Your DM is an extremely generous DM just for letting you use your Int score for your Will saves. That alone is an incredible advantage for your PC. So you get to cripple your PC by giving him a bad Wis but an awesome Int, and that basically doesn't matter a single bit because now you even get an
extremely good Will save & other than that, Wis isn't going to effect you much....so your 7 Wis is meaningless (it may as well be a 1). That's pretty unfair to other PCs unless they get the same kind of advantage.
Did you bother to argue with your DM that maybe this new feat is overpowered? I doubt it. So why are you going to post here in hopes of getting more ammo to use against him so you can pester him about his decision about the Leadership feat? That's obviously your intent.
I say, state your case with him once by showing him the Cha penalty applied for the feat. If he still says, "I still don't like it" then say, "damn, ok" and leave it at that. He sounds like a generous DM, so you don't want to be annoying him. You could make an offer where you'll pick a new feat in place of the "savant mind" feat if he'll let you take the leadership feat. That way you'll take that penalty to Will saves but gain a cohort instead. Maybe you can convince him that your PC needs a care taker because his mind is so unstable. But if your Will is giving you a +7 from Int alone, then that character doesn't have an unstable mind.
Yeah, you should get your DM to check out this thread....mainly so I can convince him to take away that "savant mind" feat cause it's way too overpowered (mainly only because you have a +7 bonus to it at level 7).