Hospital discharging patient one day out of ICU

This may be malpractice: it depends on whether it was negligent, or unreasonably careless, to discharge your son then in that way. (It seems to be so to me; but you will need a medical expert opinion as to the appropriateness or inappropriateness of the treatment before proceeding.) If it was negligent, then it likely was malpractice; if it was malpractice, your son (if he is an adult) or you (if he is a minor) could potentially recover compensation, IF the malpractice caused his later problems, for:
1) Additional medical costs caused by the malpractice--i.e. the out of pocket (not paid by insurance) share of the ambulance, the several additional weeks of care, and the medication;
2) Lost wages, if any;
3) Some amount for "pain and suffering" for several weeks of hospitalization and if there is any lasting impairment or effect.
(You or he can't recover anything for him "almost dying"--the law does not provide compensation for what "could" or "might" have happened, but only for what did occur.)
As noted above, however, not only must the medical care have been negligent or careless in some way, but you need medical testimony or evidence (e.g. the medical expert's learned opinion) that the bad care caused the later problem. If there was no causal link--if your son would have collapsed the same way no matter what--there is no liability, because the negligent care did not cause the consequences your son suffered.
A good way to proceed would be for you and your son to meet with a malpractice attorney; many such lawyers provide a free initial consultation to evaluate a case, and you can ask about that before making an appointment. The lawyer can evaluate the strength of the case (i.e. how likely you would be to win, or at least get a good settlement) and what it might be worth, and then advise you as to how to proceed if you want to go ahead with legal action.