First after examining his bedroom after 26 years, Judy & Peter walk in and Alan asks them who rolled a 5 or an 8? To which Judy tells him that Peter did. So obviously Alan is aware at this point in the film that the kids have found the game Jumanji and have played it to the point where they had finally released him.
Later, he is still fully aware that Judy & Peter have played the game because before the mosquito tries to attack them in the car, Alan asks them "What came out of the game before me?"
Then when it comes down to the moment where it looks like it's only going to be Judy, Peter & Alan playing the game, with Alan acting only as an observer and Judy's piece won't move after rolling the dice 2 times; Alan observes the game board and notices that 2 of the pieces belong to Judy & Peter and then asks "Who are the other pieces?" FIRST QUESTION: How does he not remember playing the game 26 years ago? Since it was him playing the game/rolling the dice & reading the clue that got him sucked into the board game in the first place. Yet, right after remembering that the elephant piece was his, he says to Judy & Peter that "You're playing the game I started playing in 1969". Well, Of course they are, Alan. That's how the kids got you out of the game in the first place.
Then only after Judy tries handing him the dice so he can take his turn, does Alan suddenly remember who the other game piece belongs to, yet just a minute ago in the film he questioned who the 2 other pieces belonged to (His/Sarah's)
Just seems like one minute he remembers the clue he read before being sucked into the game, but yet forgets his own game piece being the elephant and Sarah being the other piece aside from Judy's & Peter's. And it also seems like one minute he knows & is aware that Judy & Peter have started playing the game thus the reason why he (Alan) is now finally released from the game after 26 years, but yet is so shocked & surprised when he realizes (after remembering his elephant game piece) that Judy & Peter have started playing the game that he started in 1969.
And yet he totally reacted with fear because he knew who/what exactly was about to come out of the game and come after him.