With machines you get what you pay for. I would get a decent quality machine with a decent # of features,(My minimum would be:fixed clamps, 6 point suspension mounting, linnear tension head, decent brake system), but that is me, many here like the inexpensive bare bones type of machine, just not my cup of tea.The features makes the job more enjoyable for me, therefore I can string more comfortable and do a great job. A great job also can be done on a low end model as well, it's just how you get there.
If your son has a # of racquets you would not really need a portable unit. The machines on a stand are the most comfortable to string on.
The USRSA has a good article here on machines:
http://www.racquetsportsindustry.com/articles/2008/08/2008_guide_to_stringing_machin.html
And on this link, same article but features table page, if you click on the manufacturer, it takes you right to that manuf. site.
This can give you more information for an informed decision. Good luck.
http://www.racquetsportsindustry.com/issues/200808/200808stringing_machines.html