I am a fully trained first aider for almost 30 years (and CPR trained for a few more than that from beach lifeguard training too). ALL my training has been strong on the fact that there is a less than 50% success rate with CPR normally, partly to do with technique and the fact it is very tiring, but also that the outcomes are poor for the patient in many cases if they have gone into cardiac arrest and even if in hospital with a defibrillator there is a relatively low positive outcome.
What your DS has done is to keep the blood and oxygen flowing around the body of the patient as long as possible until more trained help arrives with more equipment and better drugs to keep working on the patient. So he has given them every chance of being able to respond to that higher level of treatment when it arrives - and that is much higher as a result than if nothing had been done.
I have not personally had to do CPR on a patient - lots and lots and lots of training over the years, and I now teach it to others. But a colleague did have to do it at work, unsuccessfully (and I was the person who dealt with her in the aftermath) and that was tough on everyone.
Your DS has done something physically strenuous.
He has done something emotionally difficult in dealing with a person dying (and he may not have had that experience before either).
He may be getting praise for "having done a good job" (of doing CPR and keeping going until paramedics etc arrived) which is merited for the action but he may be feeling guilty or upset that what he did was not enough for the patient. So there can be a swirl of all perfectly valid emotions and feelings.
There is probably also an element of needing to relive it through the paperwork that is probably needed (in triplicate) for work etc, and the various people who were there debriefing and discussing once professionals left and then the others who were not there at the time coming later or today and wanting to know all about it (sometimes to know what's going on and needs some kind of action, sometimes out of concern for the people who were involved, and some just feeding the drama or looking for a story to be able to tell).
Let him get out what feelings he has, they are valid.
But also make sure he knows he did a good thing and sometimes good things are not enough for a good outcome for very many reasons.
Try not to overwhelm with questions and concerns.
But give him space to come to terms with it all, allow him to talk if he wants, and if it continues to be a burden for him, there are supports available - there should be a system in work, but he could definitely ask the local hospital or red cross or similar if there's someone he can talk to.