Don't do wieghts. No good boxer uses wieghts. (Sometimes they flip plates, or use plates for neck exercises but I'm talking bench pressing, curls, etc.) I would stick with push ups, ab exercises, pull ups, and running. Running is really the most important thing for a boxer. Everybody will tell you road work is the best. I read a book by Joe Frazier and there was an entire chapter about running. He ran 10 miles a day in work boots. As for explosiveness in the arms just work out in bursts to provide trauma to the area you are trying to work. For punching speed shadow box with wieghts.

Do heavy weights with low reps. This stimulates fast-twitch muscle fibers responsible for speed and explosiveness. Also, incorporate plyometric exercises.

Jeremy is spot on I agree and that is after 25 years of boxing experience and fighting pro since 1996

just make sure you do light weights with higher reps

body weight exercises are the best (pushups, pullups ect...

I'm a lightweight boxer and I'm trying to get weight training into my schedule. What exercises are good for strength and explosiveness but won't make me move up in weight classes? Would P90X work?