RE: U-CONTROL
A1Rench,
Try your midsized (.09 to .19) planes with .012"×52' stranded steel lines to start. That's the most common length used for those planes. The kevlar lines will work but tend to be very draggy and give a somewhat soft response to control inputs. Hopefully, you still have a handle or two around.
I respectfully have to disagree with Bud about fuel. If your older .15's were well broken in and well used, and they are of the iron piston/steel liner variety, DO NOT use off the shelf R/C fuel (especially Morgan fuels) in them unless you want to start replacing parts or engines soon! You will clean out the varnish you built up using the all castor fuel and rapidly lose compression and front bushings, especially if you have a slightly lean run or two. You are correct that you should look for a fuel with a large percentage of Castor oil as the lubricant.
Most of the older .15's operate best with 10% nitro and 25% (or slightly more) lube, at least 75% of which should be castor oil. A good Castor oil to use is available from SIG. Some Byron Fuels have castor oil as part of the lubricant. SIG still makes and sells 100% castor fuels, as does Powermaster (Ask for the GMA Blend of 10%/29%.) If your local hobby shop doesn't stock and can't or won't order your choice of them for you, you can obtain them from the source in case quantities. If that is not an option, check for other CL Flyers in the Louisville area who might help you with a tank or two of fuel to allow your son to try the planes and see whether he may have a continuing interest. Unfortunately, you may not find any of those Louisville guys here yet
Have fun.