Should I stop giving my baby his antibotics for scratchy sounding throat and slight outbreak of rash on head?

Yes. The antibiotic (especially amoxicillin) may be the cause for the rash or the rash may be from a viral infection which is also causing the scratchy throat. Antibiotic does nothing for a viral infection and excessive use has led to bacteria resistant to nearly all antibiotics.
Maybe. If you feel your child is having a reaction to a medication you should call your pediatrician (or whoever prescribed the medication) immediately.