Rash on son

For the past few days my son has had this rash on his neck. It’s only on his neck. It doesn’t seem to be itching or bothering him. He’s about 3 months old. It has been getting pretty warm In my area lately. Ive been trying to keep him cool at night, but I have to swaddle him to get him to sleep. He has been drooling a lot because I’m pretty sure he’s teething. He also gets milk all down his neck and I wipe it off with a wet wipe. Which now I’m thinking was a bad idea. I’m a FTM so please go easy on me. I’m still learning. What does this rash look like? I’ve been told it’s a heat rash from my mom. I have a doctors appointment set up for Friday, but I’m wanting to do something now if possible. How do I treat this?