It is not evil or unreasonable to want consistency so you can plan.  How old is the SS and when he comes over, who is responsible for him?  Do you know what is preventing your DH from going to court?  Is it finances or something like he´s afraid he´ll get less time?

I don't think you are evil, I felt the same way....my DH always had 50/50 per court order, but he and BM determined the schedule that worked best for them.  They stuck to it until my SS graduated HS and my SD now 16 decided she wanted to make her own schedule...which mostly involved not coming last minute when she was supposed to, or calling last minute randomly to say that she was staying that night.  Her BM supported it, and at first DH did not want to fight it bc he didnt want to appear that he didn't want her here. Eventually though, he determined our home is most at peace and runs best on a schedule, and letting a teenager dictate what was happening wasn't working for our household.  So he did take a stand, we even offered to make a schedule week by week, but we weren't going to fly by the seat of our pants anymore.  She did end up starting to stick to the schedule for the most part, until a few weeks ago when she declared she has moved out.  I totally get it though, it's not about not wanting the child here, it's about having consideration, consistency, and knowing what to expect, for all of you, your SS included

They decide, often times on the spot, when my step son will come over. It is always changing last minute

Was it always like this, even when you were dating? Or did it change after you got married?

These parents don't even care what the inconsistency is doing to their child, so it is unlikely they would change it for the welfare of anyone else.

Stop changing your plans to accommodate their lack thereof.

I think for most people just having the simple courtesy of a monthly plan helps us so much! I would go crazy with not knowing day to day or week to week when I was going to have a child in the house. Especially if you can work it so that you have "kid-free" weekends occasionally! 

I don't understand the men that are too afraid of BM to get a set schedule! How often do you guys have your SS now? Is he afraid that he will see SS less if there is an actual schedule? 

Why does it seem that some people jump straight to "you hate my kid" when their partner is asking for a very basic courtesy?

It doesn’t f$$cking even matter! My DH and BM spent thousands in court and BM even devised the majority of the custody agreement. They don’t follow shit EVER and never have. It’s the same last minute shit. 

Their agreement outlines everything. Every holiday every school break....do you think I ever know what is going on? I can’t even plan Christmas because there is never a plan and if there is it changes.

this is because my dh is a spineless idiot Disney dad and afraid of Bm and doesn’t want any conflicts with his infant ass pre teen and teen.

Now that my girls are 17 and 14, I'm kind of letting them make their own schedule.  I figure it's hard enought that they have to go back and forth between two houses.  I'm sure my DH doesn't like it.  My SDs do stick to a schedule, they are here every Wed, Thurs and every other weekend.  My SD14 comes over every day on the bus after school though and waits for BM to pick her up.  My DH and I do have every other weekend kid free we do not let the kids come over on our kid free weekend, that time is priceless

Not no but hell no.  DH needs to extricate his head from his ass and get a CO with fixed visitation schedule.  He is holding you, your marriage and your own children hostage to the lack of structure in his parenting and time with his prior relationshop progeny.

This is also extremely damaging to his prior relationship child.  This kid needs to have stability and structure.  DH is failing all of his children and his wife with this crap.

That would drive me insane. Do they know or care what that instability is doing to their child?