I have seen here, several times, where they guy who "didn't want kids" suddenly remembers "oh wait, I forgot to have kids" once they hit a certain age. Often this is really problematic for the LBS, especially if she had given up her own dreams of having children in order to be with her partner.
On the other hand, your SO may simply be caught up in an affair of opportunity (after all, you weren't there and she was - and let's face it, phone s*x just isn't the same as the real thing), in which case, it may be a GOOD thing if he moves in with her. Raising OTHER people's kids is not NEARLY as much fun as raising your own. And if her kids are over 5, odds are they might resent this new intruder and may make life difficult for him. Instant reality check.
I think you also have to ask yourself - IF he were to wake up and want to come back to you, what would the new parameters of the relationship be? Obviously long distance isn't working - would either of you be willing to make the career sacrifices necessary to move together?
Also - a practical question about finances. Since you've been together for such a long time, I assume there may be joint assets (a home, investments of any type?). If so, you should get immediate legal counsel about how best to protect your share of your joint assets, since you don't have the legal protection of marriage.
