A crossbody bag works for a teenage boy who actually goes places and needs to carry phone, wallet and other essentials without it getting lost or requiring constant attention. The strap should be adjustable and comfortable over extended wearing. Material should be durable and practical because these bags get used heavily. Interior organisation should include pockets that actually help rather than creating confusion. The design should be understated enough that he will actually want to carry it regularly rather than feeling self-conscious. A good crossbody bag is practical infrastructure rather than a fashion accessory, something he will use constantly.