The companions do show up on eBay from time to time, the most recent being a used one that was posted as a "buy it now" for $40 sans box and instructions. That listing was back in mid-March. So there is some hope on that front.
Now, that said.... I don't know if you know your way around electronics or not, nor how big and complicated a layout you have, nor your son's age and level of interest in the hobby, so this might be overkill, but...
My recommendation would be to look into upgrading to a more advanced DCC system, preferably one with a computer interface. A free, open source program called JMRI (Java Model Railroad Interface)
http://jmri.org/allows you to more easily program the various settings on your decoders, and - via wifi - allows you to connect your smart phone (Droid and Apple) to your layout as a throttle. I believe that it works with many tablets too.
As for what DCC system to upgrade to... if you know your way around electronics (or are game to learn) there is DCC++
https://github.com/DccPlusPlus/BaseStation/wiki/What-is-DCC--Plus-Plus which is basically a do-it-yourself DCC system. If that low cost option is enough to turn your hair gray, you could go with the basic version of most any other DCC system. Unfortunately, it doesn't appear that Dynamis (nor EZ Command) can be connected to a computer / interface with JMRI.
I understand perfectly the need to stick to a small budget, but think of it like this: EZ Command is a new driver's first car. It gets you were you need to go, but it's short on amenities. By upgrading you get things like automatic transmission, power steering, a CD player in place of that 8-track player, and "AC" isn't just another word for opening the windows and driving really fast. A more advanced (and yes, more expensive) DCC system will let you fiddle with the CVs, power / speed match locomotives, and so much more.
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