What's wrong with him?

My one year old brother always has the worst luck when he gets sick. Every time he's sick it gets so bad he has to get rushed to the emergency room. But this time we can't figure out what's wrong. He doesn't want to eat and he is always moping around like he's sad. And he starts breathing really heavily for no reason and when he starts walking he starts losing breath. The doctors say he only has bronchitis but i think there's something more. Please tell me if you think you might know whats going on or if you've seen these symptoms before.