Againt he "i wont sell for less" is a very bold statement to make after 3 years of having a for sale sign outside of your house with not 1 view!
I also think you and your husband should debate it between yourselfs. Simply get say.. 5 reasons you think you should such as freeing up money, obviously more manageble and such things as the phsycological effect it can have. Moving a house and reuilding a "home" to what you want it to be can be an extremly pleasing experience and young teens love the whole " MY NEW ROOM!" thing!
On the other side, find exactly why your husband doesnt think its a good idea and more importantly, as your 15 year old what he/she thinks. If its to a location where they will have to switch schools or stop seeing their friends id stongly suggest postponing it untill he/she is about 18 and then maybe your husband will have a better mindframe about it all too :)