If you ordered the Valbazen... it's a very effective broad spectrum wormer, not just for gape worms. IMO, all poultry keepers should have it in their parasite arsenal. I would not give him the Valbazen if you are positive there's a respiratory issue going on tho, wormers can be hard on a birds system, especially when they are already compromised by some other issue.
Since your birds sounds congested you might want to isolate it, just in case it has something contagious that can spread thru the rest of your flock. Clean and disinfect the coop, and all feeders and waterers etc. It could be something as simple as too much dust/pollen in it's environment, but it could also be the start of something viral that can infect every bird on your property. For starters I would eliminate the dust possibility, add poultry vitamins to it's water and apple cider vinegar (2 TBS per gallon of water), feed it lots of greens and some high protein treats, and make sure it's eating and acting well/healthy otherwise.
There are a lot of different antibiotics used to battle respiratory problems, but not all are effective against every disease/illness (and antibiotics are not always needed for the bird to recover). If you can find it locally and quickly there's a product called VetRx (it's like a general poultry remedy, used for multiple issues) that may help your bird fight off the cough. If the bird is coughing up mucous out of it's mouth or nostrils... then ideally you'd want to take the bird to a vet and have them diagnose exactly what the bird has, and go from there. I can't tell you exactly what your bird has or which antibiotic to use, but if you just start dosing the bird with antibiotics that aren't going to help clear up the problem that could end up making things a lot worse.
There's a lot of threads about respiratory issues in the Emergencies / Diseases section here on BYC, maybe you can get more help from reading thru the threads or posting about your bird's cough there.
Good luck, hope your bird improves and it's nothing contagious/infectious.