Actually we have, I said something to him when I took lunch at work...and he waved his arm at me (in a dismissing way) and walked off...
If I were in your shoes, I wouldn't even think about getting a bigger house until this is resolved. Getting a bigger place as of right now would only be putting a small bandaid on a big wound. Bigger places equal more room to aquire more stuff. Then, you would be back to square one.
To make up for the small yard for now, the dog can be exercised daily (which is more important than having a big yard. Rotties are a working breed that need something to do every day ), and the kids can be taken to a park, playgroup, etc.
Now, if your goal is to eventually get a bigger place and to TTC #3, then sit down with your SO and make a plan of action. Set a timeline when certain areas need to be cleaned out, and that it needs to stay clean for X amount of time before you start looking for a new home. "If we want this, we need to do this in order for it to happen." Point out that the more stuff you have to move, the more it is going to cost you. Then, if he wants to do it bad enough, maybe he will make an effort to change his habits. But, ultimately, he is going to have to want to change - you can't make him do something he is not willing to do.