I think whether or not you tell him you're filing depends a great deal on how you think he'll take it. I knew AXH would blow up, even though he was living with some one else; there's no way I was going to tell him I'd filed. Your AH may handle it differently.
I told DS, then age 6, that Mommy and Daddy were not going to be married anymore, but that we would always be his Mommy and Daddy and will always love him just the same way we always have. We spoke about how Grandma and Grandpa love him and are part of our family while they live in different houses, etc. We would read 'It's Not Your Fault Koko Bear', 'Was it the Chocolate Pudding' and 'Megan's Book of Divorce'. The Sand Castles and Joint Custody With a Jerk books that Peaceofpi mentions were great resources.
Talking about dad's addiction... As Pohsfriend points out, disparaging remarks about the other parent are a huge issue in divorce cases.... Any discussion about the other parent's addiction needs to be done in a non-blaming, no finger-pointing way. I think, as a parent, it was my place to talk to DS about his father's illness. AXH certainly was not going to. DS heard, and hears, enough lies from his father; I'm trying to make certain DS can trust me to be honest with him. However, I didn't talk with DS about AXH's addiction until I ran it by my therapist and then DS's. And the discussion did not contain any of the grimy details about what AXH did, just a discussion about addiction as an illness and behaviors.
A book for young kids on alcoholism that DS preferred is The Brown Bottle by Penny Jones.
Note, however, that AXH's addiction wasn't the main point of contention in our divorce; his abusive behavior was. So discussing alcoholism wasn't strictly taboo. And while DS and I have spoken about why our house was always dark when we lived with AXH, and why I'd jump 10 feet with sudden noises, I have not brought up the word 'abusive' related to AXH when DS is around.