The bigger issue will be why he wants to change custody.  No court is likely to let a minor change homes "just because", they will explore the reasons. Are there problems in BM's home? Does he think the rules are less strict in your home?

How old is SS? If he's an older teenager they will take his opinion into account more than if he's only 12. But the rules are different everywhere, so look them up for your area.

Normally 12, but that doesn't necessarily mean they are just going to okay whatever the kid wants. It's just one factor.

As another has said, its usually around age 12 and the judge will either ask questions to the teen or get a GAL to do it to find out why the kid really wants to swap houses. I just wanted to add that the probablity of your DH getting primary residential is slim to none because a judge will not go from BM primary to DH primary; think of it as going from one extreme to the other. If anything shoot for 50/50 first and then after a few years if the kid is adament about just visiting BM for weekends then go back to court to fight for that. 

It's going to vary by state.  In my state, the judge will consider a child's wishes at any age, but it's not until age 14 that their wishes are given more weight in the decision.  Even then it's just one factor.

I only know how this works in one state, because I am in the middle of it. My very alienated SS14 "decided" that he didn't want to ever see his dad again last Oct/Nov. We don't know why still. However when we went to court the judge did talk with Ss to find out his reasoning. The judge stated that since this is a very dramatic change and SS could not answer basic questions on Why, he was assigning a GAL. From there the GAL makes his recommendations, writes the order, the judge signs it. 

There has to be a valid reason that is serious for there to be a change unless both parents agree to the change. In the state we are dealing with there are 13 points of consideration. The child's wishes are only 1 of those points.  

 Generally there is not defined age where a child can decide. Some states have a point where the child’s preference is given more weight but even that’s not completely set because of maturity levels.

Either way the child must be able to clearly articulate why they want the change.

At the same time in older teens there hits a point where many judges will stop caring if the child isn’t showing up for visitation and they might not even hold a parent in contempt at that point which is just sad in my eyes.

What age are we talking about? 12+ is the loosing point