Call your vet and find out if you can up the Benadryl dosage to 1 and 1/2l. I've given Wellie 3 pills at once before and my vet said it was fine. But he weighs over 60lbs.
For now, I'd wait another day. Give the Benadryl every 8 hours and see if the hives are gone by tomorrow. If they're not, I would take him in for a steroid shot.