Is it possible to sue the guy responsible for my son's injuries?

My understanding is that Illinois follows a modified comparative negligence model. That is, a Plaintiff is barred from claiming damages if he/she is found to be 51% at fault for his/her own injuries. Applying that here, let's assume that the altercation was the result of negligent behavior. Was your son's negligence 51% of what got that ball rolling? If so, then he would not stand a good chance at a judgment in his favor. Now, many altercations (fights) are not considered negligent; but rather they are considered to be intentional torts. The facts leading up to the altercation are usually determinative. Why did this guy hit your son? Was there a history between them? Was this guy (intentionally) getting revenge for something? If it is determined that the 24 year old battered your son intentionally, then that is both good news and bad news. The good news is that it would prevent the modified comparative calculation from coming in and possibly barring your son's claim. The bad news is that, if you bring your claim and win, there is not likely to be any insurer to pay the claim. Insurers are much more likely to pay for negligent behavior than for intentional torts. As for the landlord, that's even tougher. What duty did the landlord have to your son to not lease to a felon? Are there special facts that would impose a duty on that landlord to keep out felons? What kind of felony was the guy convicted of? Is this the type of incident and is your son in the class of persons that a don't-rent-to-felons policy would be designed to protect? On your limited facts, you might have a valid claim against the 24 year old; but you need to weigh the practical economic viability of taking this thing to trial. I know that the above is not what you really wanted to hear; but it's what you need to hear, I think. I am sorry for your son's horrible injury. I hope his recovery goes smoothly and quickly.

Sure, it's possible, the question becomes one of collection - that is, who will have the money to pay any judgment that you may receive. Intentional torts are almost never covered by insurance and it's unlikely the tenant has many assets. An action against the landlord may be possible as well, but the collection issue exists there too. Disclaimer: This answer is provided as a public service and as a general response to a general question, it is not meant, and should not be relied upon as specific legal advice, nor does it create an attorney-client relationship.