Walikum salam
to the forum I am sure you will find much help here :D
In my opinion you should get married asap! Because one should hurry to get married and not delay it for too long because opportunities in life come and go. But that dosent mean either that you marry blindly! Ofcourse take the necessary steps to investigate etc.
As fo your son, I believe that rather than center everything around your son (i.e. the decision to who should move) the decision should be taken by you and the fiancee based on other aspects such as which area is good to live in, more islamic, safer, etc etc. The reason I say not to base it on your son is simple: Kids can adapt easily to changing environments since they dont have financial obligations etc and depend solely on their parents. Plus kids dont have an idea really of where would be a good place to live.
But for the parents moving can be a financially taxing event also they have to look at the job market and lots of other aspects as well. So best discuss it with Mr. fiancee when applicable.
Bro Aku