Wow, B., this is a big decision to make.
You have a lot of good intentions, and all in the right places. Personally, I would tread very carefully if you decide to go forward with this venture. By carefully, I mean written contracts and agreements with your ex-partner.
Here are some potential problems, though, that I see on the horizon:
1. You will be in some state of emotional limbo for the duration of this arrangement. He's not 'there' and he's not 'gone' either. No room to grieve a failed relationship, which in my opinion merits grieving. This can also be very ambiguous for the kids, and not give them room to grieve, either.
2. "He is financially irresponsible, I could not depend on him from week to week let alone from month to month." This should be a big red flag. He's let you down even within the space of an invested relationship, and you know this guy better than we do. You are better off with a living arrangement you can afford solo. This way, if he decides to invest his money elsewhere than rent and bills, you aren't left in a bind. Ditto should he decide to move in with a girlfriend who is wild about World of Warcraft and the idea of being a step-mom. (They're out there!)
3. This isn't really helping *him* to grow up to be a good role-model to his family. He needs to man up and be truly responsible. If you aren't married and getting divorced, please seek a separate decree of child support as soon as you are able. His children deserve it, you deserve it, and HE deserves the opportunity to grow up. Otherwise, you are just prolonging the enabling and codependence. Maybe it's time for him to attend some financial counseling?
4. When do *you* get to move on with your life and stop taking care of this man? What if you meet someone terrific, who's the whole package, and then how hard will it be to move your ex from his cozy little nest within your home? He'll have it made too easy. There's no motivation for him to leave. And if he decides he wants to date... well, you've already stated how you feel about that.
5.Protect the amicable relationship you do have with him now. I think I would have a problem having a roommate with these traits. Do you think you will still feel so benevolently after watching him waste his time and blow his money while you are working your butt off? You mention being in a better place financially, but you are the only person working toward that goal, from what you describe.
I think you can really love somone as a person (not romantically) and want what's best for them, and let them know that it's time they began to take care of themselves as well as doing their part in the responsibilities of having children. This arrangement sounds like it primarily benefits him by allowing things to be the same-old, same-old. As for your youngest, while I haven't worked with kids with autism, children tend to like things pretty solidly clear. I would ask a professional (if he's in an early intervention program, or even the person who performed his evaluation) what might be a good way to help your son to understand this. Staying together, too, may give more hope than you'd like to your daughter. I think he could be a great dad, but this is not the kind of relationship you really want to keep modeling for any of your kids, right?
Sorry it's such a toughie... Hang in there!
H.