Just a suggestion, Is there any thing you can do with him other than lounging him.

I had a filly once that was well behaved never did anything aggressive, Was just like your guy as far as no matter what I introduced she excepted and then we would move onto the next thing. I always worked her in the round pen or on a lounge line. At times she would do what your guy does and just stop and come in and stand. Anyways to make a long story shorter. We put her under saddle she did great, at this point when it came to teaching her from the saddle I had some else ride her. She was fine the first few times but after that when you would enter the round pen she would be fine for about the first five minutes and then start acting up. After about a week of this she reared with the rider on her back and walked back and broke through the round pen. (no one hurt) So the next day back we went to do more work and after about 5 minutes she started up again. So we decided to take her out of the pen and just work her in the paddock area. She settled right down and went back to her usual pleasing self. We ended up figuring out that she didn't like the round pen or even lounging (we must have over done it, so to speak). We kept here out of the round pen and off the line for a couple of weeks, and never had another problem with her. I did use the round pen and lounge line with her after that but never day after day and only for short periods of time. If I introduced something new and she was fine with it, then we would move out the the paddock area or the arena. Sorry to be so long winded.

He may be getting tired of doing the same thing. Maybe you could try working him on obstacles, or try long lining him. I don't know if you are able but could you pony him with your gelding. He may just be getting bored.

The aggression does need to be got under control, but firguring out what is causing may fix the problem.

Just some food for thought, if I think of any thing else I'll post it.