Weird rash

So my son has developed a weird rash on his legs. It started as just a few bumps on one leg, then by the next day the area got a little bigger and so on. it has now spread to the other leg and is starting to spread to his stomach. At first I thought it might just be a hear rash, but he has had it since May 30th and it seems to just keep spreading. I took him to the Dr this past Friday and she told me she didn’t know what it was and prescribed a steroid ointment for him just in case it’s just a reaction to something he may have come in contact with. We have been using the ointment since Friday evening and it still seems to be getting worse. She told me if it’s not better in 10-14 days to come back in, but with it getting worse should I go ahead and take him in or just wait it out and see if the ointment starts working? He doesn’t seem bothered by it at all. No itching, fever, loss of appetite or anything.

This is when it first started:

& this was taken tonight

Any idea what it might be?