I cannot disagree with the other fine advice the other attorneys have offered. Since you do not have a lease to rely upon, the Court will create a fictional one for you (yes, even at zero rent level). This fictional lease will be deemed to renew every month. Thus, you will need to give him a thirty (30) day written notice of eviction.
On my website www.ZippToCourt.com I have a Zipp To Research tab you can directly access here: http://zipptocourt.com/Zipp_To_Legal_Research.html
Scroll down to the Illinois Landlord/Tenant law or just click directly here for very helpful guide from my law school, SIU on general advice for landlord/tenant law in Illinois http://zipptocourt.com/uploads/Landlord-Tenant_Rights_in_IL.pdf
In my practice, which is in Lake and McHenry counties, I have had several clients in your situation. The bad economy and the sense of entitlement of some of the younger folks these days have often led to situations such as yours. If you follow the PDF, you should understand what you need to do. If you would rather this come from an attorney, give me a call. This is not a exceptionally complex situation, and will not break your bank. Sometimes having the letter come from an attorney with the necessary 30 Day Notice being hand delivered by you not only addresses the situation of the freeloading child, it also restores the obviously missing respect that your son is not showing you.
This answer is marginal legal advice and does not constitute an attorney-client relationship. Every client and case is unique. The best advice is to always consult with an attorney. Free legal resources at www.ZippToCourt.com
The primary question that needs to be asked is whether your son deems his residence to now be your home. If so, you probably have to go through the formal eviction process. If he is merely a guest, then you need not allow him back into the home. You should probably consult with a local landlord/tenant attorney so discuss your options.
consult with an attorney. The previous answers right on the money. You could have serious problems if you do this wrong.
If you found this Answer to be helpful, please mark it as such. Remember, however, free advice is worth every penny you paid for it. This is only generalized commentary on your question. It is not to be taken as legal advice. I am a lawyer – but not your lawyer! Even "in person" interviews leave attorneys with plenty of questions – the Internet makes it crazy! Thank you Chuck Watson 217.544.6165