We are pretty much on one income and we have 1 biological and a foster child. I work part time and use our church a lot, that has helped so much! Our church offers Parent's Day Out, drop in care 2 days a week. They are very reasonable, compared to other child care/daycares. We have used a sitter who has her own home daycare and that has helped keep the cost down. The big daycares are more expensive, we've found due to their overhead. I would try finding someone in their home or use your church as much as possible. You can find someone in the home/outside daycare by checking with Campfire. They offer a service of helping you finding childcare for free and if you need more help they charge some for it.
We used to be on 2 incomes, however I have a disability and I'm no longer able to work full time anymore. We've had to cut back big time.
We've learned to cut back on a lot things since we have had kids. Another idea is to check with friends and see if you can help each other out at times. I've heard of Moms doing that with other Moms. Starting a group, especially in the evenings, where if you want to have a date or go somewhere, your friends that want to help out would watch your kiddos and then do the same for you.
You had mentioned your Son, has Downs Syndrome. We've been members of the ARC. That would be a great resource for you and your Son. They might know of other ideas for childcare and saving money too. They also have meetings/group each month for support and general info on different topics. There is one in Fort Worth that meets at the Child Study Center that is great.
A flex schedule would be helpful, if your husband could be home more or change his hours any at all. My husband is now home 1/2 a day on Fridays every week. I set our appointments that day, if we can so we don't have to get a sitter the other days or use his sick leave. Good Luck