E.B.
A couple of suggestions: ask to speak to a social worker or patient advocate or representative at your youngest child's hospital. Tell them your 5 year old is having difficulties handling having a sick younger sibling. Ask for a referral for help for your older son. There are classes, therapy for kids, counselors, play groups, all kinds of resources for siblings of sick children. It may be at the hospital or they may know of a therapist who deals in that subject.
When we would have to take my youngest child to the hospital or ER, and my older child would have to come along, I had a small bag packed. It held some really cool toys that were only for bringing to the hospital. One was a fold up little garage toy with a little pretend elevator (a Fisher Price thing, I think). There were some Legos, and some juice boxes and small snack size boxes of crackers. The little garage had a handle and when he was done playing it folded up like a little suitcase. I'm sure they still make plenty of that kind of thing. Or maybe have a special video game like a Nintendo DS or something, that only is for bringing to the hospital. I have memories of my youngest on the hospital bed and my older child underneath building a Lego village, happily entertaining himself. Sometimes I'd put a special new little toy in the bag.
I also put my older child "in charge" of little things. Like having a notebook and writing down his sister's room number (I knew it, of course, but he felt like he was "in charge" of that kind of thing). And I let him keep the parking ticket stub from the parking garage in a secure pocket. He would have to find out what his little sister wanted brought from home (her blankie, a toy, etc) or what she hoped to eat when she got home and he'd write it in his special notebook.
It is normal for an older child to state jealousies ("he's so lucky he's sick") because of the attention given to the younger child, or to be angry, or to act out, or even to wish that they had a disease. When those kinds of statements or behaviors occur, simply stay calm and say "I wish all my kids were healthy" or "I'm glad that ____ has good doctors to help him feel better, and a family who loves him". Don't get into a debate or say "you don't mean that" or "that's mean". It is not clear thinking when they're that young. Simply phrase your response in a way that you hope your older, healthy child will eventually be able to state for himself someday. Be the empathetic and caring role model for your healthy child.
When my older child was in middle school, his teacher called me one day. She said there was a sick child who came into the class for an hour a day or so, as part of an educational plan. He had an oxygen tank and was very weak and sickly. The teacher asked for the kids to pair up for some kind of fun poster project, and she noticed that the sick child was left out of all the excited pairing up. My child walked up to him and said "You wanna be partners?" The teacher wanted me to know that my child was a real encouragement to that other child, and said that she noticed that my child didn't seem afraid or hesitant to just treat him like a normal kid. I told her my child had a very sick younger sibling and she said she had an idea that some situation like that existed from the way my child acted. It had been years since those days of constantly going to the ER and the hospital and he had long outgrown that silly little garage toy, but all that time there was kindness in his heart.
So please look at this from a positive point of view. It's a chance to develop awareness of disabilities and illnesses, a chance to demonstrate love and kindness, a chance to instill in your other children a sense of empathy. You may not see the results for several years, but it's growing all the time.
I know it's hard to have a sick child and a healthy one.