RacingHeart: I know several home schoolers. The good ones are awe inspiring. The others worry me. It is a terrifically challenging thing to do.
And your criteria are...?
Do you love love love sitting and helping him with homework? Does he love love love working out fun ways to solve dull homework problems with you because you inspire him so much? If not, what's your starting point for assuming HE will be a solution?
Fortunately, majurormi (as you probably know), the above is irrelevant as you don't need to help with homework or solve dull homework problems while HEing.
Are you trained as a teacher? ...
Again, this is irrelevant. Home educators do not need to "teach subjects". Majurormi can learn along with her DS as they do things that interest him.
What steps can you take to ensure he doesn't become socially isolated?
He already has outside interests - leaving school does not mean giving up everything else.
how will you monitor his progress to make sure he doesn't fall behind?
When my DD decided to try school for the first time at Y5, I went to WHSmith and bought some NC workbooks so that we could check how she would cope with maths and English. If Majurormi wants to check his progress in terms of the NC, it is not difficult.
Not just academically but socially and emotionally. Can he really jump back into school at 13 having been out of it for two years?
Most HEers report that their children have no problem re-entering school - two years of building confidence, interacting with all the people they meet as equals and enjoying themselves tends not to have a negative impact on young people. He may, however, have a lower tolerance for the irrelevant nonsense that goes on in school (e.g. petty uniform rules, name-calling, "typical" teenage behaviour).