Amoxicillin sensitivity

So my little guy apparently/possibly has an amoxicillin sensitivity. He just ended his round of 10 days yesterday and today broke out in the craziest rash. It will appear and disappear and pop up in different places. I broke and gave him Benadryl and it cleared it right up. Thing is he is only going on 18 months, and our pediatrician said to give him the Benadryl if we need to. I gave him 1tsp as instructed at 12:12pm. It is now 3:24 and the rash is coming back! Has anyone dealt with this before?!

Before Benadryl

This is not even the worst of it

After Benadryl

Now

Any advice?! The pediatrician said not much we can do besides Benadryl, but I don’t want to give it again if there is anything else we can put on it to help.