Rooster, but more like 3-4 months old, I think. The age thing is throwing a lot of folks off, but if the bird was born in April, it would basically be full grown by now and not growing appreciably as the OP has noticed. The darker shading on the wings was my first sign, the saddle feathers are not really bold yet but are there, plus those thick legs. The rose comb can throw a lot of folks off also, that bird isn't a a rir/wellie cross or it would have a straight comb. Most common rose comb breed is wyandotte, cold possibly be a buff wyandotte with a rir, or just a barnyard mix from the sound of the seller.
I'd love to see pics in 3-4 weeks to see how he looks then, if you keep him. If/when you're sure he's a roo, see if the seller will make it right for you.