It looks to me like Rainbow Servant doesn’t advance familiars. I was expecting Mordy to, around now, try to convince Milo to please take more Wizard levels so he can talk to other rats. Since that doesn’t look likely, will Mordy ever advance more?

I can’t imagine what rat you would want Mordy to talk to…

There are some tricks to advance familiars despite being in a PrC, but the easiest one takes a feat. Milo’s most recent feat choice was made in a moment of panic, and not with foresight as to what would be most useful, or keep him from getting bit by Mordy.