I was looking at the details of one of my current opponents, he's played 14 games so far and won all of them (but has a few in progress).

But the bar chart of his ratings shows two sequences of four consequetive games where his rating continually drops. How can he lose points when winning a game, even if against inferior opposition?

Is it something to with his provisional rating 'maturing'?

Yes, I think it settles down once you have completed 20 games.

If you have a provisional rating and win a game against someone whose normal rating is 400 or more points below yours, or whose provisional rating is 200 or more points lower than yours, your rating will go down.

See the FAQ for more details.