My son is 9 months old roughly between 17-19 pounds and he had 1.25ml of fever reducer about 3 hours ago, is it ok to give him Benadryl (diphenhydramine)?

No infant Benadryl (diphenhydramine) Best not to give Benadryl (diphenhydramine) to your 9 month old for a fever. It may make him sleepy, but is easily overdosed, which is why cold medications are not recommended for infants anymore without a doctor's prescription. Use sterile saline drops in only one side of the nose at a time to help clear secretions. A cool mist vaporizer in the room. Lukewarm sponge baths if high fever and see your MD.