If you can't keep him away from what's causing his allergies (grass, trees etc) then wipe him down every time he comes in. It should help a little.
I would also say give him baths more frequently to keep him free of the allergens.
You can also try the regular, unflavored Claritin. For a 60-65 lb dog, it's 10mg every 24 hours. It's helped one of mine with hives. I know some here also use Zyrtec but I can't remember the dosage @ddnene