I don't know about his mouth swelling like that but I completely agree with the allergies! I have a boxer/bulldog and he rolls outside constantly in the summer. He's soo itchy and after rolling around in the yard for awhile his back is covered with what looks like " bumps" all over. I always use the benadryl and of course the oatmeal bath and with this combo it seems to help pretty good! :) Also (I'm not sure about where you live) fleas are bad this time of year here so I'd try to get rid of all 3 just to be safe.
I would stick with the benadryl, and try oatmeal when you bathe him. There are some really inexpensive oatmeal products for pets, but, good old quaker works, too. Make sure you rinse, rinse, rinse after bathing. Rinsing is so very, very important. Good luck. It is allergy season, and pets have allergies, too.