Switch to Active Duty & MOS

I've searched but can't find anything current about this specific issue. My son is set to go into reserves as LAV crewman. He really wants tanks, but there's no nearby reserve unit for that. He is in college now, and he sort-of wants to continue for awhile (other days, not so much). Right now the decisions are (1) change & go active duty to get tanks or (2) stay reserves & try to switch to AD & tanks in a year or two after some more college. Questions: If he goes reserves for LAV, exactly what is the process if he wants to switch to Active Duty AND switch to tanks? Is it a realistic plan? Does it take a long time? Is it likely to work? He definitely wants to be active duty ("real" marine) but there are some good reasons for him to wait awhile.