To just add what everyone else has said, quite often it will appear to get worse before it gets better. Even with benadryl it will do this.
I had a dog that had an allergic reaction to something that I couldn't find out what it was, I took her to vet and she got a shot, and I was told to keep her on the benadryl. It got worse the next morning and I took her back again, and the vet told me that it can happen like that. And she was miserable and looked like she had jelly beans under her skin with red spots all over her body.
I would take her to the vet, but do try the benadryl. Now this is just me but IMO I'd wait and see what the benadryl does, if the swelling gets worse I'd then take him in.