There are a lot of things that can cause a rash. You might google "rash on child" and look at some of the pictures, but you are probably better off taking the child to the doctor and finding out what it is.
could be an allergic reaction too. i agree, look at pictures online and seek medical attention. is he itching? a good idea is to outline the rash with a marker and see if it is growing or shrinking. hope your little one is better soon. i wouldn't worry about baby yet.
personal experience with searching online: turned out my mom had shingles and she had one day left to get to the doctor for medicine. hurray for the internet!