R.K.
I think your heart is in the right place. And you are absolutely right about fearing his abandonment issues. My mother got divorced when I was a baby and my brother was 2 1/2. My dad just took off. He would come back from time to time until my brother was 4, then left for good. I was the lucky one - too young to know I had a dad. My brother never recovered. Even though we had (have) the best, most loving and devoted mother in the world, my brother grew up angry with her, thinking she made dad go away. He has had issues all his life and at age 50 now, he's beginning to understand how "off" his thinking was all those years. And by issues, I mean low self-esteem and lack of respect for authority, which led to touble, obviously.
I think your stepson's mom is a bit in la la land. She has a fiance whom she is probably totally into and has mistakenly put this new man as a priority before own son. She may never win Mother of the Year, but if she is not abusing or neglecting him, even if she doesn't make a lot of money, I think the boy is better of with her. Who cares if it's just a one bedroom apartment. The kid is little. I'm sure he'd much rather sleep on a mattress on the floor of the livingroom than not be with his mom. Also, on the positive side, they are saving money by being in a smaller place. As the child grows, they will be inconvenienced enough to work toward getting a bigger place. There are plenty of poor people in this world raising more than one child in one bedroom shacks. It may not be ideal, but as parents, we can't just decide we can't afford to keep our kid right now (unless they are out of work and homeless).
Also, since you are bearing the brunt of Clark's emotional turmoil over his mother, you and your husband have every right to address these issues with the mother. You can lay down the law on this. She has to know how her leaving him and not knowing when he'll see her again is devestating to him and it is unfair to treat him that way. At the very minimum, until he moves back in with her, there has to be a set schedule of when she will call and when she will visit. He needs the comfort of knowing he'll at least talk to mommy at 8:00 every night and can look at the calendar and count the days until her next visit. That's only fair.
Good luck with this. You're a GREAT step mom. Let us know how it works out.