I assume Grunfeld. In fact, with 2.c4 played, I don't see how this could transpose into the KID. As I thought that c4 isn't played since I've seen 1.d4 g6 to start.

The KID is 1. d4 Nf6 2. c4 g6 without ...d5 (whereas with ...d5 it's a Grünfeld). 1.d4 g6 is the Robatsch Defence, which is distinct because Black having omitted ...Nf6, White can follow up with 2. e4.

Then I guess that c4 is played which is a different type of transposition that I'm just not used to.

This, along with the Four Pawns Attack, used to be my favourite response to the King's Indian Defence.

I played through the score from the club forum, and formed the impression of rather passive play by Black.

I agree with ION and find this move pointless.

Not wholly pointless; it develops the knight, avoiding ...Nc6 as White could then biff the knight with d5.

Perhaps this Black likes to play provocatively, is confident in his defensive technique, and prefers to induce in his opponent an unsound bellicosity. Perhaps.

I suspect he didn't know what he was doing. He played the first 4 moves quickly and then started thinking quite a lot.

I'll bet he was expecting you to close the centre (d5), then go for a Yugoslav style of attack (g4, h4, etc, after suitable preparation, of course).

Perhaps he ought to have waited until the bishop actually got to g5, then smacked it. This move has a consequence worse than what he feared...

The consequence. Castling short will now be difficult to arrange. Does Black want to play ...h5? I don't think so, as one White plan might then be to close the centre, then start shoving his g- and h-pawns.

I was wondering if he may castle by hand by playing Kf8-g8-h7

Superb centralisation!