Half of me agrees, the other half doesn't know.
I think he can have a bigger pair, like 7's or 8's, but anything bigger would have raised pre, no?
Given his position as SB, seems there are more combo's of 9's in his hand than pocket pairs... 6 combos of each pair, so if we assume only 7's and 8's, that's 12 combos of likely better pairs. But many combos of 9x in his hand: 92
, 92
, 93
, 93
,...
But maybe there's a chance he's betting smaller pair, which is likely from SB...