Hi
Welcome to IL.
You have mentioned that your son is a GC holder. A lot of young people get married before they apply for their GC's cause once if they get the GC and then get married the waiting time for the spouse to get her GC is very long. I have heard that it takes over a year and may be even more. Hence she cannot come to the US along with her husband. So it is probably easier if he gets married to someone who is already in the US either a citizen, a GC holder or even who is in a H1 visa. Anyway you can check their website and find out more details. Hope this helps.
Regards,
Jothi.