See his doctor. Any insect bite causes a tiny break in the skin, allowing bacteria to get in and cause a local skin infection. One such sore in an older child without fever may respond to topical antibiotic ointment, but a baby with a potential skin infection should be seen to make sure it is not more serious. Sores resembling spider bites may suggest mrsa, a bacteria that needs special antibiotic treatment.
Show it to a doctor. If a mosquito bite is infected, the doctor will usually prescribe oral antibiotics. So, if a baby has a bug bite that looks worse or bigger than the average little mosquito bite, the baby should be seen by the doctor.
Get it checked. If you think a bite, or any other small break in the skin, looks infected, have it checked by your pediatrician. Some skin infections can be serious. In the meantime, apply warm compresses, keep the area clean, give Ibuprofen for pain and Benadryl (diphenhydramine) for swelling or itching. If the bite is extremely red, swollen, painful or draining pus, have it checked right away.
Call your doctor. A mosquito bite (or any skin lesion) that appears infected needs to be examined. If your doctor agrees it is infected, they will prescribe the appropriate treatment.