Tabby isn't a breed, it's a pattern, so anybody can give a solid answer on that one---yep, he sure is! Very definitely. Does he have stripes on his sides or swirls? If stripes, he's a mackerel tabby, if swirls, he's a classic tabby (also could be a spotted tabby but from what I see he doesn't look too spotty ).
Definitely tabby. I love those brown tabby boys. My particular brown tabby, Tiny, is better at understanding me than any other cat I've ever met. He's practically psychic... only, y'know, not; just very perceptive.
Yep, "tabby" is a pattern that occurs in many breeds, as well as in random-bred cats. I would say yours is a random-bred cat, since he doesn't look particularly like any breed, but does resemble the domestic cat's wildcat ancestors.
Here, for example, is a photo of a wildcat, the species from which we believe our cats descended. You can see the brown tabby pattern, which looks a lot like your cat's. This particular cat is an African wildcat.
When a cat doesn't have a breed, we just call it "domestic longhair" or "domestic shorthair" for the purposes of the vet paperwork; realistically, though, DLH/DSH is a huge category, with so much variety and beauty that you can only recognize a DLH/DSH cat by comparing it to known breeds and finding no matches. About 95% of cats are DSH/DLH.