You can't really vent sex chickens after they're a few days old - the vent is no longer pliable enough to open far enough to see what's in there. Judging from the size of this bird it's probably possible to make a good guess at the sex from other characteristics. Could you post a picture of it in profile, with the comb clearly visible?
With that much wattle development and redness of comb, that would be a roo at 8 to 9 weeks of age.
I've only see the "business" of a rooster once...when I happened to be bending down to pick up something off the ground and looked up at the opportune moment that a young Barred Rock was defecating. I could see, what I can only term as, extra "junk in the trunk" that I recognized was different than my hens. And yup, that bird turned out to be a full fledged rooster.
But I agree. The vent is simply not pliable enough at this age to vent sex easily and the roosters "spare parts" are not easily distinguished as they are fairly small and well enveloped inside the cloaca.