If he is doing fine on motrin and it has brought the fever down -you don't want to eliminate it completely why add tylenol? He should be able to have another dose of motrin at 8.
http://www.drgreene.com/21_832.html
A fever does not necessarily need to be treated. If a child is playful and comfortable, drinking plenty of fluids, and able to sleep, fever treatment is not likely to be helpful. Steps should be taken to lower a fever if the child is uncomfortable, vomiting, dehydrated, or having difficulty sleeping. The goal is to bring the temperature between about 100 and 102 degrees F -- not to eliminate the fever.
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I was surprised to learn in medical school that fever, far from being an enemy, is an important part of the body's defense against infection. While a fever in a child signals to us that a battle might be going on in a child's body, the fever is fighting for the child, not against.
Most bacteria and viruses that cause infections in humans thrive best at 98.6 degrees F (37C). Raising the temperature a few degrees can give the body the winning edge. In addition, a fever activates the body's immune system, accelerating the production of white blood cells, antibodies, and many other infection-fighting agents.
Cold blooded animals adjust their temperatures by moving to a different spot. While a medical student, I sat fascinated as I learned that lizards, when infected, will move to a warmer place to give themselves a fever. If they are prevented from finding a warmer spot, they will likely die.
Many parents fear that fevers will cause brain damage. Brain damage from a fever will not occur unless the fever is over 107.6 degrees F (42C) for an extended period of time. Many also fear that untreated fevers will keep going higher and higher, up to 107 degrees F or even more. Untreated fevers caused by infection will seldom go over 105 degrees unless the child is overdressed or trapped in a hot place. The brain's thermostat will stop the fever from climbing above 106 degrees F. Some parents fear that fevers will cause seizures. For the great majority of children this is not the case. About 4% of children, though, will sometimes have seizures with fever. These febrile seizures are caused by rapid increase in temperature, not by the height of the temperature. Once a child already has a high fever, a febrile seizure is unlikely with the current illness. In any event, febrile seizures are over in moments with no lasting consequences. Treating fevers early in these children may prevent further febrile seizures.