Sorry if I repeat, but yes, it is due to the illnesses that they can be a vector for. There is a whole slew of diseases they can transmit. The variety your son might have been exposed to depends upon the variety of tick (and there are several types of those... depending upon what part of the country you live in).
Best practice is to "save" the tick. Typically, after removing it gently with tweezers I drop it into a small ziploc bag and add alcohol to preserve it. Then I apply neosporin to the bite area. Even if they do not transmit any diseases (lyme, etc)--and those may not show up immediately---the bite itself could still become infected since the skin has been opened.
I typically retain the tick for about 2 weeks. If, during that time, your child exhibits any signs of illness, then you still have the tick if you need to have it tested to see if it carried any disease.
Lyme disease is but one. (and the one that sometimes produces a red ring around the bite area, although the red ring doesn't appear until days later).
Many are insidious, in that the symptoms are similar to those of many other potential causes, and do not appear immediately. Some can have no symptoms appear, but still cause damage, and no symptoms appear until there are serious problems.
There are other serious illnesses:
Ehrlichiosis
Rocky Mountain Spotted Fever
Babesiosis
Anaplasmosis
and more, again, depending upon where you live and what kind of tick it was.
In any reading I have ever done on ticks, I seem to find that everything says that there is rarely any transmission of disease until the tick has been attached for over 12 hours (most things actually said 24 hours). So if you are certain of when he picked up the tick, then you may be able to relax a bit. If you cannot pinpoint when he was first possibly exposed, then keep an eye on him for signs of illness that might be related.
http://www.cdc.gov/ticks/diseases/
This one has specific information to the ticks found in Illinois:
http://www.idph.state.il.us/envhealth/tickkey.htm