My baby is 9months have had a fever since last thursday and have red patches all over hus body any idea of what thus could be?

Roseola? If the fever lasted 2-3 days, then went away and the rash appeared, then it is from roseola (a viral infection). If fever and rash are present together, then a doctor needs to see your child as the possibilities are too many to answer here.
Infection,see md. Please see pediatrician to determine if this is bacterial or viral infection which is accompanied by a rash. The combination of fever and a rash is highly suggestive of an infection amd should not be ignored.