First of all, I'm glad to hear that things are going well by you. And you're always welcome to drop by my thread!
As far as you brother, I can only share my thoughts - I'm far from an expert. I will say that I'm touched by how much you love your brother and want to help him grow!
Your first instincts are correct - ultimately, we daven. We can do our hishtadlus, but remember that that's all it is - we're doing actions in this world that we're supposed to do. Ultimately, you can't control what your brother's doing, and trying to do so may not end well for anyone - he may end up resentful, and you feeling frustrated.
I think, though, that you can bring it up - Start off by telling him how much you love him and care about him, and how much you respect him, and are impressed by everything he's doing. Then, you don't even have to mention what you saw, though you can if that feels right - but discuss the idea of kedusha, keep it short and simple. Explain that sexuality is very important, and also very holy, but should be saved for the right situations. Mention the idea of a filter, explaining that you also have a filter because you know how difficult it can be for anyone to control themselves when they feel urges.
I'd recommend that you not say too much about your own struggle, though you can let him know that because you're human, you realize that when the rabbanim talk about how important it is to have a filter, to prevent us from falling, you realize that it applies to you, too.