My mum was diagnosed with Fibro, Chronic Fatigue and Spinal Arthritis when she was 40 and I was 4. She retired (Law clerk) and became a stay at home mum. My childhood was different from my friends in that my mum didn't go to work and couldn't participate in many school things (selling raffle tickets, any candles or flowers- she is allergic to most "chemical" smells) but my childhood was great because I knew what was going on and our bond became stronger because of it.
From day 1, mum was 100% honest with me in a way that I could understand as a child. I learned more as I grew up (I'm now 18) but what I knew made it easier to understand what mum was going through. I have watched my mum go through Fibro flares that have made her muscles knot to the point she couldn't get out of bed, and I've also seen her run around the backyard with our dog seemingly without a care in the world.
My advice would be to begin by explaining the basics to your son. Poobie's advice is a great place to start for an 8 year old. Him understanding that what's happening is not his fault will give him peace of mind that he doesn't even know about! (I went through fazes at the beginning thinking I was a bad child for making my mum tired). Have conversations with him about how you are feeling on a particular day or if you have a muscle knot that prevents you from doing something let him feel it (lightly) and teach him "gentle hugs". Knowing that mummy is sick but will be "okay" is a great relief to most kids.
One of the things that made my mum and I so close was our after-school talks. I'm not sure if you are still working but mum retired when she was 40. Everyday after school I would come home, make her a cup of tea (in the microwave) and we would talk about my day for anywhere from 15 minutes to 2 hours, rotating between sitting on her bed (her laying down) or the couch depending on how she was feeling. This kind of interaction is so valuable because it sets a routine that can be altered if you are having a good day or a bad day. Seeing you in whatever state you are in also allows your son to ask questions about what you are experiencing and gives you the chance to help him understand. I don't think childhood is defined by what a parent can do with their child but it is about being present with your child.
With today's technology, you don't need to miss any performances your son has. Kids know when their parents aren't at a performance and it hurts if they don't know why. If you are too poorly to make it, have a mother of one of your son's friends film the event and watch the tape back with your son later. This will allow him to give you the highlights and for you to encourage whatever he is doing (school recitals, baseball games, plays). One of my favourite things to do as a kid was showing my mum when I did something, whether it was a drawing or a dance recital.
As a kid, I did take on more roles than most of my friends, not because I necessarily had to, but because I wanted to. While not all kids are the same, help will come in age-appropriate steps. For me I started making tea in the microwave at 5, on the stove at 8 and in a kettle (mum finally bought one) at 12. My steeping and milk skills evolved over the years as I grew up.
As I went through my pre-teen years, I did get angry sometimes that I couldn't go to certain things because mum couldn't drive that day or couldn't take me. I soon realised when mum told me she was sorry but she couldn't change what was happening that I needed to think of her feelings as well.
Your journey is something that your son should be involved in. It removes some of the barriers these illnesses cause by bringing you together. While some people think it robs them of a "normal" childhood, I personally think my unique childhood was awesome! I am closer to my mum than most of my friends are with their parents and steps ahead in life skills (I was doing grocery shopping at 14 and moved across the world for uni at 16).
I hope my perspective as a child of a parent with these disorders help. It may seem daunting now but it will get easier and you remember to "Just keep swimming" :)