Don't change medication doses or combine medications without talking to your allergist/family doctor first. Benadryl may help with the skin reactions, but again, check with the doc to make sure that it's OK.
I just want to say, though, that there comes a point where you just have to accept a certain level of symptoms. If it's affecting his sleep, his schoolwork, or his favorite activities, it might be time to go back to the doctor and try some stronger meds. If he's wheezing, it's time to go back to the doctor.
But you're probably not going to be able to medicate every symptom out of existence, especially the nuisance stuff like itchy eyes and skin. I've been walking around with that every spring and fall for most of my 54 years. Could I make it go away if I took prednisone for 6 months out of the year, or Benadryl round the clock? Probably, but I wouldn't be able to function. I just deal with it. My meds are focused on the symptoms that really impact sleep, work, or activities - wheezing, sinus problems. It tamps them down enough that I can work and enjoy life, but I still feel lousy enough to take a sick day a couple times during allergy season. It just goes with the territory.
The ads for meds like Zyrtec and Claritin and Flonase make it sound like they're magical things that make every single symptom go away. It doesn't work that way. They can go a long way towards reducing symptoms, for sure, but for many people that's about the best they'll do.
And I know that moms don't want to see their kids in any discomfort at all. I really do understand that. My mom still wrings her hands when she sees me with red eyes during tree pollen season.
Now for some useful tips......it's great that he's getting a bath before bedtime, that's really important. He should also wash his face when he comes in from being outdoors. Even changing clothes can help, especially if he's been playing outside and on the grass. If you have a pet that hangs out indoors and outdoors, brush them frequently to get pollen off their coat.
Cold, wet compresses help a LOT with the itchy eyes. Just wet a washcloth with really cold water, squeeze the excess water out and have him put it on his eyes till the itching calms down. An aloe cream around the eyes helps a lot, too, as the skin gets irritated easily.