The bigger the boat - the more difficult things become. Just as an example - docklines. Big steel
boat - very heavy boat - docklines will be minimum 25 mm (probably more) 25mm docklines weigh in at say 1 pound per meter. Now think about carrying a 40 meter line around (40 pounds - can you hold/swing 40 lbs in 1 hand?)
a 50 footer is not easy and requires a lot of practice. So doing it single-handed is extremely difficult.
PM Dockhead - he has a 54 footer - I know he thinks singlehanding
is very difficult.
Better to stick with something around 40 feet. This can be handled and is spacious enough for you and your son (if not - then you probably shouldn't go sailing).
I sail a 40 footer can do so singlehanded, although when docking
in strong winds, I'm very happy indeed that we are two persons