Moms don't like to admit it, but we usually pay more attention to the younger child while the older one is burdened with more responsibilities. Your situation is more complex than mine, but when my older daughter accused me of making her do more than her little sister, what she needed to hear was that I do it because I trust her, and her little sister is just too young. It hadn't occurred to her that she has the more important duties because she's strong, trustworthy, and capable, but our conversation helped her understand, and now she protests less. You'll have to tailor the talk to your 5-year-old, but a good place to start is to explain to your son that there will always be a little inequity with your time and responsibilities because his brother's brain is different and Mom and Dad need to do things for his brother that they don't have to do for him. Assure him that you don't love him less and your heart is big enough to love them the same. Then follow it up with action: Make sure that you aren't relying on your 5-year-old to be "perfect" because he doesn't have the same issues as your autistic son, and give him plenty of one-on-one time so he feels special. Your child will understand how loved he is, and respond in kind.
Q. My 5-year-old is jealous of his 3-year-old autistic brother. How can I help him understand we love them equally?