Some thoughts...
Hello!
The vet will probably do a skin scrape, alot of times it will help explain whats going on with itchy scratchy dogs! Also, mange (like demodectic) is quite common among short haired dogs such as bulldogs, boxers, boston terriers etc. and can become generalized if not treated, which could explain why it seems worse this year. But there would probably be hair loss involved if it was mange, especially if it is genralized, and would be pretty obvious, so it doesnt sound like that is causing it.
There are some dogs who are indeed allergic to grass, my friend had a golden retriever who was and she couldnt be outside for long periods of time and had to take her walks in the street/sidewalk strictly because she would scratch so much she'd scratch off fur and create sores and scabby areas. There was even a great dane who was allergic to sunlight i read about, and in such cases the best option is to once again talk to your vet, who might be able to prescribe a cream or ointment or even a pill to help with the allergies.
It is also possible that he is getting bitten by bugs. My boxer gets terribly bitten by mosquitos and flys in the summertime and often times bumps will show up where the bites are and he'll lick and bite at them. If your boxer was allergic to the bites he might even show bigger lumps and bumps. I use a bug repellent that is made for dogs that works pretty well, but i'd double check with your vet in case the repellent could irritate his skin more.
Does he have dandruff? A skin scrape would would tell but its possible he has mites, which can be easily transferred from dog to dog. Or just dry skin, in which special shampoos or a change in diet might help.
Hot spots are more common in dogs with long hair and undercoats but any dog can get them to varying degrees so that could be another reason. However like mange it would probably be more obvious as the spots usually turn into sores that sometimes leak fluid.
Sometimes, even just brushing everyday can really help because the dead skin cells and hairs can build up and be itchy, especially when your dog is shedding more during certain times.
Its good you are going to the vet tommorow! My boxer had mites AND mange (which we luckily caught early on) and he was a pretty scratchy guy too. These thoughts are just my best guesses, but i am sure your vet will have a solution. Best of luck, i know how frustrating it can be trying to pin point the cause of itching and scratching!