At What Fever Temperature Should I Take My Child to a Doctor?

107.2 fever in a boy 3 months old with downs and heart problems.

ANY individual with a temperature of over 105 (accurately measured, which in a 3 month old child should be rectally) should be evaluated by a doctor.

A child with Down syndrome, coupled with cardiac disease, and a temperature of 107 (I wonder about accuracy here) needs immediate and thorough evaluation.

I would suggest immediately speaking with your child's doctor.