I have to honestly say that I would not take Boomer to a ballgame. He is a nervous kitty and I don't think he would do well. It might not be so bad if you could go home immediately if things didn't go well but I don't think I would chance it. If you do decide to do it I would strongly suggest you also take his carrier with you. A freaked out cat on a leash is not going to be controlable and great harm could come to you and Boomer. If you had a carrier to get him into quickly you could at least avoid injury to you both.
Missy I believe with all my heart that you are trying to help Boomer and that you have nothing but the best intentions but some kitties are nervous and scared of things no matter what we do. I know your goal is to desenstize Boomer but I just don't think it's possible. He is what he is and you love him just like that. We all know you do.
I hope you don't get angry at what I said but I'm trying to be honest and give advice based on my kitty experience. Some cats are just jumpy and nervous and stay that way their whole life. I'm pretty sure that is the way it will be with Boomer. You can't find the rhyme or reason-it just is.
I really, really strongly recommend that when you take him out on the leash near people for the first time that you have his carrier on hand. Trust me on this one! If he does well then no harm done but I promise you if things go bad you will be very thankful that you had it!
Good luck and keep us posted on Boomer's training!!

Hope your not mad at me cause I think you and Boomer are great!