what is the best reserve branch?

My boyfriend wants a career in criminal justice but since he is not 21 he cannot start his career for two more years. I am waiting a year to start college for in state tuition and he just says he doesn't have money but wants to go. I thought that joining reserves would be a good option for him considering he wanted to join the army before anyway but I am wondering which branch offers the most incentives with a military police job perferrably.