This situation can get complicated. First, it is advisable (from a family perspective) to ask yourself whether getting the legal system involved is worth the potential fallout of hurt feelings and mistrust that may result. Second, the "legal" answer is muddled without more facts. Generally speaking (and there are exceptions), an eviction is only needed where a "landlord-tenant" relationship exists. This is usually evidenced by an agreement of some sort wherein you provide a place to stay in return for something (usually rent). Otherwise, whenever someone's welcome is up they become a trespasser and can be removed by police.
Now here is where it can get complicated: if a person stays in a place in a certain amount of time, and they refuse to go, then the eviction statutes may be invoked and you may have to go through a formal eviction process in Illinois. This is because "self-help" is prohibited in certain cases, even absent a landlord-tenant relationship. This legal principle is from the old days of squatters and adverse possessors, but a sharp defense attorney may use these statutes against you.
I would recommend sitting down with an attorney to hash out all the details before you act. They will be able to apply your facts to the law. I would not in any circumstance throw your son and his belongings outside. You may (if he is angry enough) be liable for damages as a result.
This is a tricky situation, and I wish you luck.
NOTE: This answer is not intended to be legal advice and should not be construed in that way. This answer does not create an attorney-client relationship and no such relationship may be created absent a signed retainer agreement. The author is licensed in Illinois only, and his answer is for educational purposes alone.