Watching anyone having a fatal heart attack is a horrible experience to go through, let alone watching it happen to a loved one.
It is hard to say what an individual feels when it happens to them, and i'm sure it is different for everyone, but first up, let me say that he probably did not experience suffocation.
It sounds like it went very quickly from "discomfort" to passing out. It would've been sudden and I'd say he was unconscious and not aware of what was going on. Rest assured that he'd know you did everything to save him, and sometimes there is just nothing that can be done. It is often the case that the first sign someone gets of a heart problem is the first time they ever get any chest pain. You did not doanything worse for him and did the best you could under the circumstances!!!