First off, hi there and welcome.
Back to the topic, what foods have you tried? We're not a big fan of Iams here, I believe it's a 1 star food. I highly doubt that the switches are doing damage, more so he's getting tummy aches from the changes. We recommend slowly transitioning him into new foods, for example, 20% new food and 80% old food for the first couple of days, then change to 40% new and 60% old the next 2, and so on.
We find that a lot of bullies here are allergic to chicken and grains. So you'll notice that we suggest Fromm beef frittata to start as it has neither of those ingredients. But a lot also has been doing well on Natural Variety Instinct salmon and Avoderm.
What probiotic are you using? Also, what is the antibiotic for?