Sorry, I realize you won't like this, but the situation is beyond your capabilities. I also realize that you want to ride a horse badly and this is your only choice, but believe me - you are not ready for the issues that you might run into with this horse. And he doesn't care that he's 25 or that he has lived with you for such a long time - given the opportunity, he will dominate you. And it will hurt.
And you should separate the horses. He can really hurt the mare and the foal.
I have come a long way, to surrender my shadow to the shadow of my horse.