I have the Always, Clarity, Catalyst trilogy on my hard-drive, and I don't think it's what you are looking for. Brian does call Justin's son "Sonnyboy"--with an "o"-- just as he does Gus, and Lindsay is jealous of Brian's relationship with Finlay, Justin's son.

However, Lindsay does not specifically object to Brian's using the same pet name for both boys, nor does Brian call Finlay "Sunnyboy" with a "u". I had my computer search each of the three stories for "sunny" or "sunnyboy" and it didn't find the terms--except for one reference to "sunny Boca Raton".