Depends on how high of a rating your son is at. And by decent if you mean 1300-1500, I think his rating will be down to 300 before you can get there.
I DO NOT RECOMMEND THIS:
AGAIN, I DON'T RECOMEND THIS.
Only if you are desperate, you can make a lot of random accounts and just start winning there. You can comepletly forget about those accounts later.
I have an account on my iPhone and my son has one on my iPad If I play both sides and keep beating him how long will it take to get a decent rating?