it will not relieve the rashes , but would just disinfect the affecred area. the treatment would depend on type of rashes whether it's due to viral or fungal infection. or these could be due to an allergic response.
try avoiding for a few days and check if there is any difference. babies have soft skin so avoid direct use of any product unless u r very sure about it and have tested it. u Can use dettol in the last rinse while washing his clothes.