The law you want is 735 ICLS 5/9 (IL Statutes Ch. 735). You can find it in any IL library or online here:
http://www.legis.state.il.us/ilcs/ch735/ch735act5articles/ch735act5Sub60.htm
DO NOT pay the court costs. Your son and his cotenants are paid in full. If landlord's sole basis for evicting them is non-payment of rent, your son will be entitled to costs (costs of serving answer on landlord and maybe attorney's fees) from landlord. Landlords get punished when they bring frivilous suits to court just because they have lousy bookkeeping skills. This encourages them to upgrade their record keeping methods.
So, if the boys want ot hire a lawyer to up landlord's punishment, they should feel free to so so.
If they don't hire a lawyer, they just get reimbursed for what they have to pay a service of process co. to serve landlord with their answer. If the answer specifies service by mail on landlord, they don't have many costs at all.
(735 ILCS 5/9-114)
Sec. 9-114. Judgment against plaintiff. If the plaintiff voluntarily dismisses the action, or fails to prove the plaintiff's right to the possession, judgment for costs shall be entered in favor of the defendant.
------------------
This is not legal advice and you are not my client. Double check everything with your own attorney and your state's laws.