I'm so sorry for the loss of your daughter. Fayfrank, and I'd like to think I've an idea how you might feel, just because it's something I struggle with myself - the world moving on, while I'm still reeling. (Even though its been 7 years in a couple of weeks)
My girl's best friend told me she is getting married, and my first reaction, inside, hidden away, was "Well, that shows how bothered you were!!"
Absolutely ridiculous, for me, but difficult for you, I know.
He does sound to be a very decent man, though, so I'm sure he will be putting that little boy first. 
I'm sure your girl would be wanting whatever makes her son happiest, too.
Anyway, what I say isn't advice, as such, just thoughts.
I'm the last person to have any answers to the impossible; I often think I'm still doing badly myself x