Maybe you can't? Its probably better than nothing but its not the way to maintain optimum muscle mass if that is your goal. Though if you're a newbie lifter (which you are if you've been lifting less than a year) then you probably don't have all that much gains yet (above your normal sedentary level) to maintain.
For a newbie lifter pretty much anything will work for a while including what he has you on. Like the other guy said its not a bad way to strengthen ligaments etc for lifting heavy weights latter on but over all not optimum I wouldn't have thought.
Find a new trainer? At any rate perhaps you should put the question to him what the theory behind the high rep work is (probably something along the lines of burn off the fat for now, worry about building maximum muscle size latter. Though some people would also dispute that high reps low weight is optimum for fat burning too).