Post nasal drip sometimes doesn't show you any other symptoms than: lack of appetite (so much gunk in his stomach he's not hungry), cough (usually worse at night when he's laying down), no energy (no food no energy). Personally I am not a fan of mucinex, even the children's dose makes my heart race so I don't give it to my kids. They both have allergies, both have nasonex (sp) I love it. It really works well for us. Also you might want to get him checked for allergies. Fall hay fever started a couple of weeks ago and it can make you feel miserable.
As far as the canker sores, I have always gotten them for a variety of reasons. When I was little I would get them when a tooth was getting loose. When I ate too much food with a very high acid content ( exp. spaghetti sauce or a bag of oranges, fresh lemon aid ect.). Later in life I got them when I first started my period every month (just giving examples I remember you have a boy). Now I get them if my body is off for any reason, too much acid foods, not enough vitamins ect. While I have never tried B-12 for canker sores I've heard it helps. But the best thing I found is an OTC vitamin/supplement called Lysine (you would probably need to split the pill for the right dose) It seriously works well for me so it might for you. Salt water rinse will help too. And, if the canker sores get worse don't wait to get them checked out. Mine got so bad a couple of times I had to go to an ENT to have them burned closed in my mouth. Just keep an eye on them!
Good luck!