Kevin confirmed that the Second Movie rumors were true, but as of this moment he is still waiting to find out how much of his character (Poseidon) will be in this production
So that's why I read the book;-) and I'm afraid there's not much physical Poseidon but there are many connections to his character.
The Book is a good read, now that we have gotten to know the characters this one revolves more around them embarking on an adventure wrapped in the well known Greek Mythology story of the Golden Fleece. I could possibly see a few Voice over opportunities for Kevin's character and two appearances. (see the snips below)
"I couldn't hear them (Sirens), but I could see they were singing. As their mouths moved, their faces morphed into people I knew - my mom, Poseidon, Grover, Tyson, Chiron.
Then my dream changed. I was following Tyson to the bottom of the sea, into the court of Poseidon. It was a radiant hall filled with blue light, the floor cobbled with pearls. And there, on a throne of coral, sat my father, dressed like a simple fisherman in khaki shorts and a sun -bleached t-shirt
Book is available from amazon
click image above for a sneak peek of the first few pages