My baby develop yeast rash on his neck, back and buttock for about a month now what can i do to treat it?

May not be yeast!: That pattern is more typical of seborrheic dermatitis. "yeast" medicines won't help, but anti-inflammatory ("cortisone" related) creams will. If your own doc is having trouble with this, you might see a pediatric dermatologist. Keep in mind that to diagnose your infant accurately, we'd need to know his age, health status, & what has and hasn't worked. Actually seeing the rash helps us too!