My son had covid & got the original virus. he was hosptilized.11 days, almost died & came home with oxygen. wouldn't his antibodies be very high now giving him a natural immunity to the virus?

Natural infection does give those with a NORMAL immune system a robust natural immunity to the stain that infected them. New strains may be able to infect those with natural immunity but the severity is more likely to be less. Vaccines developed against the original covid may or may not add additional protection from the new strains or reduce their symptoms if they become infected.

Get him vaccinated, if and when he is eligible. Get booster doses as advised by your doctor. "Natural Immunity" is not sufficient.