Let Him In
Poor Fiancé. He has a hard row to hoe here, coming into an established family, trying to assess who's for him and who might not be, and all the while desperately trying to make a good impression on everyone so that your daughter will still love him when they leave your home. After all, if he can't fit into her family, will she still want him? Will she come to see him in a different light if he's the only one who can't keep up with the dinner conversation?
Try to remember what it's like to be the outsider. You went through the same thing with your wife's family at some point; how did that work out? Were they rotten to you, establishing a tradition of you avoiding her relatives at all costs? Or did they welcome you and accept you better than your own family ever did, thus instantly doubling the number of people that you consider kin?
You might feel as though nature should just take its course. If the boy fits in, he fits in, and if he doesn't…you can't force it. That's true to some extent, but there are ways that you can help ease his transition:
Avoid overwhelming him. Don't try to introduce him to too many family members at once. He's trying to get to know the immediate family right now; keep it simple.
Prepare yourself with a little knowledge of his interests and his line of work. If he works for a power company and you know nothing about it, educate yourself so that you can have a meaningful conversation.
No inside jokes at the dinner table. Even if your family relies mightily on little sound bytes in order to sum up certain life events, inside jokes are designed to keep some people in the loop and others out.
Keep a positive attitude. Be prepared to give him the benefit of the doubt for a while. If six months pass and you're getting nowhere with this man, then you're free to make a negative judgment. For now, cut him some slack.
If you've done everything you can possibly think of and the relationship still doesn't get off on the right foot, at least you'll know that you gave it every chance. People are different; you might not ever see eye-to-eye with him, but you don't want to assume that from the moment you meet him. Give yourself — and him — plenty of time and opportunities to accept each other, and your relationship will be as good as it can be.