Wait for him to crow.... even pullets will "chest bump" and spar some... the females will generally back down, while the roos will keep at it....
You will see the tail start to look more like a rooster's tail.. and the feathers of the neck will change.. the colors on the neck and wings will get new definition and color..
You will know for sure when / if he crows... (not at weeks old, at months old)
More red, dark comb and wattles.. larger comb and wattles along with deeper / brighter red...
That chick is older than six weeks.... imho.
You will be able to tell when they are more like four or five months old. A picture of "him" with the others.. all of them together, outside... might help... other views, more pics.
Not dark red comb, that I see... and no wattles.. light around eyes and such. Might be different than the others, but I am guessing pullet still.. so far. Have to wait and see.