Remember, though, that Rider is also a familiar. A slightly unusual one, yes, but a familiar nevertheless. And, unlike Ruu, she's still contracted to her original summoner and protective of her.
Yeah I didn't forget, although Ruu doesn't even know what's a Servant, let alone they have a Master. For now, he only met Archer(Alice's) who's still bond to a Master and didn't pick up on the kind of relationship they have. 'Look like friends' would be his assumption for now. So, unless he gets a clear explanation(he's a bit slow), there's little chances he would understand by himself that Rider is a familiar, especially when her Master isn't around yet. Kuro and Kiyoshi, on the other hand, it is easy to understand.
(honestly, I'm surprised he isn't upset by Shuya too)
Only my guess but, might have to do with Satoshi saying he wouldn't bother with him, which is probably not a bad choice.