should we give water to 4 mo?

Kat
My baby is 4 months 10 days .... He has developed some small rashes/bumps some red some colorless ... All over his body and face, which he keeps rubbing. I have been told it's heat and I should give him water. 
Please help, I showed it to dr n she says its some allergy or something even she couldn't tell why exactly it has come.