
Bloat and Scours is a common cause of death in pet rabbits. they are the same except that in Scours the poor buny eventually ahs dir rear, which causes dehydration.
Fr some reason Gas builds up in the stomach, and the digestive process stops. It most often happens if they eat wet grass, and feeding any appreciable amount of lettuce or food that spoils quickly will do it too.
When I bred Rabbits I never fed Lettuce or Damp greens. I found one of the best greens (because yu can get them in good condition) was Cauliflower trimmings. If feeding grass only give dry long grass, never that cut with a mower, as it heats up quickly.
Rabbits make very poor patients and usually by the time they displat signs of ill health they are very ill indeed.
You did the best thing for him.