I agree with those who said that, if you sense the kid hanging back (or if you are), just respond with a high five, or a knuckle-punch (whatever that's called), or whatever. No need to say anything about it, just jump in there with your response and move on.
As an aside, I have very kind and well-meaning in-laws who also are all about facilitating family togetherness when their two sons (one is my DH) come into town. One time they kept suggesting that my oldest son (I think five years old at the time) share a room for the night with his uncle, who is a good guy, but my son rarely sees and barely knows him (he lives across the country). I responded a few times with "No, that won't be possible", and did eventually add a few other, and valid (but not the main) reasons why. (Uncle is childless and likes to sleep, this child gets up a lot, it's not child's bed so he'll be restless, he'll be nervous, lots of bathroom visits, etc.) Finally, upon repeated requests by the in-laws, I said, "Listen, I don't want DS to sleep with people to get to know them better. Not a lesson I want him to learn." Said lightheartedly. That ended that suggestion for that child, and it never came up for either of our other two, either. They were coming from a good place, I think envisioned a "sleep-over" type scenario, but geez. I wanted to get to know my in-laws upon entering the family, but they never suggested that I sleep in a room with them to do so. Sometimes I think people disregard that kids have feelings, too.
(BIL was thrilled to have the room to himself, BTW. This was all MIL, with a hint of FIL. The discussions were quite awkward while they lasted.)