Thats awful, prayers for him and his family.
Rumor is anesthesia complications from a shoulder surgery, don't know if thats true or not. If it is there are only a few things that I can think of that could have happened as complications are rare, especially for a healthy young adult.
1) Hypotension decreasing blood flow to brain while in beach chair position (sitting up)
2) Malignant hyperthermia. When exposed to anesthetic gases or a paralytic called Anectine, an extremely small portion of the population has a defective gene and will react with extremely high temperatures (108+), tachycardia, acidosis, etc.
3) Blood Clot/embolus
4) Local Anesthetic Systemic Toxicity. Shoulders typically get an interscalene nerve block, I've never seen one have this complication, but if a large dose of anesthetic was injected into the vasculature or spinal cord instead of the surrounding tissue/nerves, the patient would be in a life threatening crisis involving seizures, arrythmias including bradycardia, hypotension, and even cardiac arrest.
If it put him in the ICU my guess would be either 2 or 4.