We last saw Tommen during the “Blackwater” episode of Season 2 when Cersei came thiiiiiis close to poisoning the lad in order to spare him from whatever awful fate awaited him should Stannis’s troops take the city. After all, if there’s anything the Lannisters know, it’s how royal children are treated during a sack. He did not appear in Season 3 and is now being played by a different actor, Dean Charles-Chapman. Charles-Chapman also played Martyn Lannister, one of the two young hostages killed by Robb Stark’s mutinous Karstark contingent in Season 3. Just, you know, to keep things super-confusing, I guess.
They sure are playing fast and loose with the recastings this season, huh?