Temperature of 99 degrees is not serious and does not require more Tylenol. However, if he continues having fevers, you may want to have him see a doctor to find out why he is having fevers. Fevers are usually a reaction to some kind of infection. Usually, it is more important to treat the infection than the fever. It also depends on the age of your child. Young children can get febrile seizures with fevers above 103 so it is important to treat them. But 99 is okay. I would just continue checking his temperature and be sure it doesn t go any higher. If he is eating, drinking, and acting okay, he is probably just fine. Hope this answers your query.