What to do when DH is denying that he even has ADD?

DH was diagnosed with ADD a couple years ago, but he does not think it's a valid diagnosis.  He was treated with Adderrall at that time and nothing else, so it didn't go that well.  He hasn't taken anything for it in at least a year or two now. 

I am now at a point in our marriage where I have to decide which is worse--being a single mom with two young boys (7 and 1) or being in a marriage with a spouse with untreated ADD.  It's that bad.  Everything I read here rings true for me--shouldering all the responsibility, feeling like a nag, feeling ignored by him, lack of sexual intimacy, he always says he'll change but never does, I sometimes refer to him as my 3rd child, he walks around in his own world, his mood swings are unbearable, he is totally inconsistent in his parenting, I feel like I'm walking on eggshells all the time, he can spout off random facts that he's read but can't remember what I told him to feed the baby for lunch. 

I am very angry and have been diagnosed with depression.  I am taking anti-depressants and seeing my own therapist.  We are not in therapy together.  I have always said that my #1 depression symptom is anger--now I wonder if my anger is in part due to his ADD (I read somewhere here that the non-ADD partner often presents with a lot of anger).

He is finally in therapy (after I threatened to leave) and has mentioned the ADD to his therapist, but it's not being addressed as far as I know.  He probably wouldn't even be in therapy if I hadn't said anything.  He said he's going to save his marriage--he would have never sought it out if I hadn't asked him to go.  Now that he has started going, he is angrier than ever.  I know he's been talking about issues with his mom.  She was just visiting us for a week and it did not go well AT ALL.  He is very upset with her, which is spilling into our life here at home.

I talked with DH about my therapy and how my therapist was helping me to see his behavior and what was going on in our relationship through the lens of ADD.  He was LIVID.  Absolutely LIVID.  He is upset that my therapist is diagnosing him based on what I say (I guess the psychiatrist's diagnosis doesn't count) and resents bearing the brunt of the blame for our marital problems.

I thought we had finally stumbled upon something that would help explain our martial and personal problems that have been going on for the last 7 years (since the birth of our first son).  I had so much hope!  But I am now at the point where I cry everyday and I am having a hard time holding things together.  His denial and refusal to seek treatment are a deal breaker for me.  Being a child of divorce myself, I never wanted to seriously consider divorce, but I CANNOT continue to live like this and still respect myself.

How do I help him understand?