Unfortunately, we have a lot of experience with this. This is the plan that was recommended by the pediatric allergist for my allergy kid:
1) non-drowsy (in theory) antihistamines (claritin, zyrtec, allegra). Different people respond to different ones in this category, so if you try claritin and it doesn't help or makes him tired, switch to allegra, etc. Try each one for at least 3 days before you give up on it. In our house, zyrtec makes us tired but allegra does not, so we go with allegra but I know people with the opposite experience so it's trial and error.
If that doesn't help, or only helps minimally, line 2 treatment: add a nasal corticosteroid like nasacort or flonase. Keep in mind that this acts as a preventative and you have to use it every day for about a week before you will see a difference. There is no children's formulation that I know of. Our allergist had us cut the dose in 1/2 (1 spray/nostril) daily. No drowsiness or anything like that with this; however, we only use this when allergy season is really bad because our allergist said there are a few reports that long-term use of these in kids can slow their growth. If kids go on and off, their growth rebounds when they go off, but if they stay on it continuously, there is no opportunity for the rebound growth (*I didn't look at the science on this, I'm repeating my understanding of what the dr said).
Beyond that, for long term control, there is always immunotherapy but we haven't gone that route for my kid (I do it for me and it has helped a lot but committing to weekly and then monthly shots for years was beyond what I wanted to do for a child).