Have one that works...
I have a Canon LiDE 30 scanner that works well with XSane. If you look at the SANE list, it is listed as having "Complete" support whereas the 3500C is listed as "Good", meaning good for some after some work. "Complete" here does not mean "Plug and Play" guaranteed without some additional packages and effort. linux is not "Plug and Play".
http://www.sane-project.org/sane-mfgs.html
Rather than fighting on with your 3500C, consider that as old gear, a scanner like the LiDE 30 or other "Complete" models can be found used at a cheap price.
The HP all-in-one machines that are supported by the HPLIP driver have a working scanner function under HPLIP, and HPLIP is included in many distributions as a standard package in their repositories.
Unfortunately, the HPLIP site with a support list is not on-line, so if you were to consider a HP all-in-one, you should google with the model number and HPLIP to be sure of support. Indeed, in the same way you might check for any scanner you bought with reference to any problems and your linux distribution.
Your son has it easy, but so could you if you started out with a device with better prospects under XSane or HPLIP.