He is probably chewing to relieve itching from the food he ate.
The stimulation of the chewing in the area of his penis may very well have caused the bulbus glandis organ that expands when a dog mates or has an erection to become stimulated.
You can see the anatomy here in this drawing
When the dog is stimulated that gland swells and the chewing could cause that as might itching along the penis.
You didn't mention what antihistamine you used. Some are not safe for dogs.
You can see a list of ones that can be used with dogs here
and a kg = 2.2 pounds for calculating dose info.
If the dose says per dog then that range is for small to large dogs.
Hope this helps you!