I think that after the events with the actor who played Goyle in the movies. (he didn't play in a lot of POA) That the WB has required doubles for each of the key actors! Neville has got his now!

Read Here!

He's also the one who confirmed the casting of Rita Skeeter!

What happened with the kid who plays Goyle? I mean, I know that he wasn't in PoA much, but is there a reason?

Either he was sick or they wanted to feature new characters coming up in the 4th movie without intros and maybe his parents grounded him from the movie? Who knows...I think it's probably sickness or school.