I'm a construction manager, and I can tell you that if all the ducts are connected, the next step would be an "air balance." There are specific companies that do nothing but this. They measure the CFMs at each register (cubic feet per minute of air blowing out) and then they adjust the dampers accordingly, until each register has the same amount of air blowing out, per cubic foot of air space, in each room. This way, all of your rooms will be the same temperature.
One way to try and do this yourselves is first of all to ensure that your ducts even have dampers in them. Many older (or less expensively installed) units do not have dampers. Your husband can try and install these himself - he can get the parts at a Johnstone Supply or other HVAC parts supply place (maybe even Home Depot). And then from there, adjusting the dampers would be a little bit of a process - maybe close off the dampers a bit in the rooms that are closer to the HVAC unit, open up the ones that are further away - play around with it until you're happy with the temperatures in each room. Considering the cost of an HVAC or air balance tech to come to your house (anywhere from $75/hr on up), it's definitely worth it to give this a go on your own first. Worst case scenario if you need to have a professional come in afterward and fix it up for you, you'd still be no worse off cost-wise than you are now.