@alwayscurious My daughter spent a lot of time in hospital as a baby with respiratory illnesses. One of the doctors said the babies outgrow these things (resp. illnesses) and are happy healthy babies, then they look up at the parents and the parents are a complete mess. It is so so hard to have a poorly baby, especially in an emergency situation. Be kind to yourself, not just about if you did the right thing, but the memory of seeing him floppy. It is going to haunt you and you will get reminders of it where you least expect it, but you need to provide yourself with coping mechanisms for this. I don't mean to sound all doom and gloom, but it is horrible being the parent of a sick child, once the shock wears off.
You absolutely did the right thing by calling an ambulance and not taking him yourself, you can't monitor him if you're driving so it's not worth the risk. I called an ambulance in the past knowing it would take longer than if I drove but would still be less risk as she could be monitored constantly and not sat slumped in a car seat that would make her condition worse. That is what the emergency services are for.
I hope you get some answers soon and you can manage whatever it is that caused it.
Give him lots of hugs and kisses.