Please help!

So my almost 1 year old had this little rash pop up. The big red spot had been there for about a week and a half. I wasn’t worried at first because it was just one little spot (the big red one) and his skin is sensitive and he’s had small breakouts before no biggie. But we’ve not changed anything at all. The only thing that’s changed is him drinking whole milk which we mix with formula still. But it’s just getting worst. He has his 1 year appointment next Tuesday so if it’s still there I’m hoping to get some answers but until then I’m coming here to ask other mommas! Any ideas? We’ve used cortisone cream for a couple days thinking it was his eczema flaring up but it’s def not helping. It doesn’t seem to bother him at all though. If you’ve got any ideas please let me know, thanks!