The data on that is still patchy, but I read through a number of articles and it seems like somewhere around 50% of patients who make it to the ICU succumb to the disease. Here's an example of an article
Data from the Intensive Care National Audit and Research Centre (ICNARC) showed that of 165 patients treated in critical care in England, Wales and Northern Ireland since the end of February, 79 died, while 86 survived and were discharged. The figures were taken from an audit of 775 people who have been or are in critical care with the disease, across 285 intensive care units. The remaining 610 patients continue to receive intensive care.
But I don't think there are firm numbers yet.
I know it's really bad to say but if he does not make it out of this then I really hope it opens peoples eyes on how bad this really is and can happen to anyone that does not follow what is being suggested by medical staff / government.