We would absolutely love that!
Without the child support and the 50/50 crap we might be able to afford the tickets for him to go see his mom...it would all have to be worked out on paper but it sounds feasible to me.
We have talked about it a few times but the state that they live in (I dont want to disclose too much) is very much a "mothers right" state, so I would be very surprised if they would let us (or better, DH) get custody of him.
We have talked to our attorney about it briefly in the past but she said unless the mother is horribly neglectful or abusive or has some sort of drug problem, the chances are slim to none.
And yes, his stepfather is a very nice man, who cares for him a whole lot and DSS has nothing but good things to say about him
Honestly, no judge will remove a child from the custodial parent unless abuse or neglect is proven or if the child is old enough to make the decision that he/she wants to live with the other parent.
As for 1st families, going to court and requesting reduced child support based on reduced income is a possibility given that the reason the income has been reduced is no fault of the parent. In that they were down sized or laid off, or became disabled. Most judges won't reduce child support if the parent chose to reduce their income. Such as taking a lower paying job, or quitting a job. The judges reaction is usually, "you knew you had a child and you knew that your support was X amount of dollars, so you should have kept the job that covered your child support.
As for additional children, it is the same story. The judge will, again, tell you, "You already had a child, you knew you were responsible for X expenses as part of your child support agreement, you shouldn't have had more children if you can't afford that, and the additional children."
I have been there and done all that. We have also been the parents that pays all the bills regardless of the child support agreement (which for reference was that the mother was supposed to pay 50% of educational and medical expenses) and all the transportation. I think it is great that your husband gives her 50% Kids are expensive. I have 4, there are always extras popping up that no-one budgeted for. I wouldn't want my child to do without if I could manage the expense, so I commend you husband for making the extra effort. I always hated the "well you got your support, use that," line. Your husband could use this line, and legally be within his rights to do so, but think, morally, it is wrong, and would, in the long run, create way more resentment than already exists. Which will eventually filter down to his son. Believe me, his mother will say to him, "you can't play baseball because can't afford it because your dad won't help."
...but I do think the trip is over and beyond that. I think when you make plans to do big family things you need to include the whole family and your husband's son is a part of the family. You really need to stop thinking of "mine" and "yours," and counting what he gets, or where he goes with his mother, and comparing it to what your daughter gets. It may seem "fair" or "unfair" in your eyes, but you are not seeing it as a child, and you are not able to be objective because your instinct is to protect your own child. ...but having helped raise the child that was left out, believe me it is hurtful to the child to know that mom/dad does these things with the "new kid" but doesn't with him/her. I understand that you do things with him when you see him, but a WDW trip is a big thing. It is not a day trip to the zoo or circus. Again, try to put yourself in the same situation, if the situation was reversed, how would you feel if your husband took your son on a WDW trip and didn't even try to include your daughter (who was old enough to know what she missed) and then say "Well, if I didn't have to pay for your plane ticket, you might have been able to go." or "we can't afford to take you." I would think that this would make a child feel like a burden.