In some cultures, children drink tea or non-caffeinated types. Caffeine of course is very not good for children or teens.
Some drink Chamomile, mint tea, fruit teas, etc. There is also "Red Tea" which has many uses like helping for eczema, diaper rash, allergies, and a myriad of other things. You can Google it. Of course, analyze it for yourself.
I have also seen at Safeway, a tea for colds specifically for children. You might try going to your grocery store and looking... or at a Whole Foods, or natural food store.
Again, MAKE SURE your child is not allergic to anything in it...since these have natural/herbal blended teas.
Have you taken him to the doctor?
For congestion, whether nasal or chest, drinking LOTS of fluids (water) is always the best.
Orange juice, will make more congestion, when sick.
Warm fluids can help loosen it.
If his throat is sore, you might want to take him to the Doctor to make sure it is not Strep. They do a quick throat swab. Strep throat was going around in my town, among kids.
There is also a baby/kids version of "Vicks" vapo rub. This can be put onto his chest. Do not use the adult Vicks on him... too strong.
The "Animal Parade" brand also makes great natural vitamins for kids, and other things.
Hyland's also.... as Deanna Leigh mentioned once, they make a great "honey" cough syrup for kids... it does work and even I used it before for myself. It's soothing. YOu can find it at any natural food store.
BUT, watch the cough (coughs can indicate a spectrum of things/anything), and for any fever.. .this can indicate a worse illness or infection. If anything progresses or gets worse, or NEW symptoms arises, I would take him to the Doctor.
Here's some links on tea and toddlers (there are many opinions on it):
All the best, take care,