First, I was wondering ... what was the flea shampoo you used?
I'd keep him away from the rest (as I would w/ any new bird brought in), if you haven't already added him. Not sure on the Tylan, but you can use the links at the bottom of my post to look it up (Merck Vet Manual would be a good choice).
For certain, I'd add Apple Cider Vinegar to the entire flock's water, at the rate of four teaspoon to every gallon of water (but not in galvanized containers). It's does many good things, and absolutely never any harm. In your situation, it will reduce the viscosity of the mucus so that it can more easily be expelled. Additionally, it helps to 'cut through' the mucus and other coatings in the mouth, throat and intestines, which improves the uptake of nutrients/vitamins, and the medicines that you may be soon givin' him (and, possibly, the rest of your flock).
When my birds stuff their fat lil' selves, they tend to carry a bit more to the right han.. wing side, towards the lower portion. And, with the water and grit, it does tend to be a bit squishy. But, these can be symptoms of other conditions/diseases, so ... back to those links ... there's one that will assist you by identifying the potential problem based upon the symptoms, which then give you somethin' to look up in the other manuals.
And, I'm gonna send you to one more link ... it's the introduction page to an online presentation on the anatomy of the chicken ... you can >
>peck here<< to open it in a new window. The actual URL is: