Should we wait a while?

What is the name of your state? Missouri
This post will probably sound familiar after reading the others regarding step-parent adoption. We're trying to find out whether my children's step-dad should attempt to adopt now or if we need to wait a while. Their bio dad mingles in and out of the picture on an irregular basis. My husband is their stability and the man they have called "Dad" for over a year now. He provides for them in every sense of the word - at all of their ballgames to cheer them on, helping care for them when they are sick or hurt, has them on his medical and dental insurance, etc. We both work and provide a decent homelife for them.

Here's our problem - Child Support Enforcement garnishes my ex's wages when he has a job. So sporadically we receive bits and pieces of child support. He is close to $4200 behind at this point. He quits his job after about 6 or 8 weeks, waits a few months and will get another one. So on paper, he is attempting to pay support even though it is through garnishing of wages.

The other part of the problem is that he only calls about once every 2 to 3 months and hasn't had them overnight since last November.

Since he is calling every once in a while and we get a child support check (which is less than what the court ordered) on occasion - does that mean he is still a part of their lives enough that my husband couldn't adopt them?