A technique I often employed with my own daughter, my grandson, and kids I've occasionally worked with over the years is role-playing and puppet shows. These can reach kids in a gentle and supportive way that no amount of verbal instruction can touch.
You might have his toys act out a getting sick / going to hospital / getting helped by doctors / coming home to his safe house scenario. Go lightly on the parts that would have been most frightening or unsettling to him, but do acknowledge them. Pretending that's all over and meant nothing just won't work – it probably all left very deep impressions on your son's sense of security.
But stress the parts where he gets better and comes home to his own safe and cuddly bed. You'll be able to see the parts of the story that calm him, perhaps make him smile or giggle. His responses will help guide you to the language that works best for him. Have his toys lie down after some good snuggles to a soothing lullaby. (It doesn't matter if you can't sing – my grandson loves our lullaby tradition when I spend an evening with him, and my voice sucks.) Kids need and like lots of repetition, so take advantage of that and repeat your plays at least once a day.
And start using that lullaby at night after cuddles. You may need to sit by his crib, patting him and singing to him for a week or three as he processes memories that are probably traumatizing to him. By doing so, you may be able to help him establish a new pattern of happier, more relaxed feelings and thoughts.