I can't comment on your kiddo as his experience is not the same as mine or DS1's.
but recently I was put on amoxicillin for a sinus infection. On day 8, during the night before I felt off and kind of yucky but could not describe what I felt, got up, took a shower, looked in the mirror and gasped in shock. From my jaw bone to my ankle bone I was one big nasty rash. It started to itch like poison ivy. Horrible. Now I have taken this med in the past and the doctor said it happens to lots of people and I can no longer take that family of anitibiotics.
A couple years ago, my son was given Keflex at the doctors office and we came home with a script. In less than one hour he started to vomit horribly and when I put him in the tub to clean him up, he was neck to knee horrible rash. I called and they had me bring him right back (they are closer than the hospital). It was an allergic reaction and he can not have that family of antibiotics. He was on amoxicillin 3 weeks ago for an ear infection and he does fine on those. I was told it happens to a lot of people.
Sorry your kiddo is going through this and I hope the augmentin works and there is no reaction to it. Aveeno Baby Oatmeal tub stuff made my rash feel better - or perhaps it was in my head, LOL! I avoided Benedryl because I needed to function and if I take that, I'm way too sleepy!