may I know.. it's true if your baby has "ruam Susu "or another elergic skin better don't bath him with soap?

because my son full body and face got spot with red colour, some people says it's to hot so baby can't thought it.. buts some people say its not suitable with soap. to much exception ? so I'm just wondering it's elergic skin or just normal for baby and it's will gone a few month or do something before its gone be bad