I'd have to agree it might not be best to take him to a ballgame right away, but like CCL said, don't let that discourage you. Start out slow, like maybe a car ride or two and see how things go. If it seems like he's enjoying it, try the ballgame, but again like CCL says, have the carrier handy in case it's just too much too soon.

My question is, if you take him to a ballgame, won't that be announcing to the world that you have a cat? I thought you're not supposed to have cats in your apartment?

Other than that, I say take it slow and let Boomer indicate whether he would like to take things a step further or if he'd rather just stay home where it's safe and familiar.