A:AnswerNot sure about trade ins, but the Apple Watch with cellular will definitely work on Verizon. The watch will function without a separate plan, however, the phone would have to be in Bluetooth range at all times.
A:AnswerI think you can either get cellular service for it with either Sprint or At&T or link it with his phone, which he must have within 10 ft from the watch. Linking with your watch is not going to help you if he leaves the house beyond 10 ft, because you will lose service connection.