His weight is just below the 50th percentile for his age. He has been through a big medical ordeal. Usually when children are ill, their appetites decrease. Suggest feeding him small meals, 5-6 times a day and giving him food he wants until he is over the bronchitis. Recommend decreasing dairy products and replace with soy milk. This may help him breath better. What is important is that you relax at meals so he is able to relax and maybe start to eat again slowly. Hoped this helped you.
If you aren't already, you might want to look at trying to feed him very plain foods and see if he'll accept any. Boiled veggies, or fresh soft ones like avocados. Plain rice, maybe. I've known of a few little ones who developed allergies around the time they started solids (or celiac disease, which means you can't have wheat, rye, or barley), and then they became ill due to the stress on their bodies and started showing avoidance patterns to their food because it hurt their mouths, or their stomachs, etc...
Not saying that this is the case, but it might be worth cutting out any processed foods and seeing if he'll accept some less processed ones?