Building muscle on an older horse...?

Recently i've heard (from a good friend/trainer) that, for building muscle, "trotting builds muscle and running takes it away". I need them to lose weight too, which before i got great results by running them for 5 minutes. I need my halter horse/pleasure horse with muscle (he's naturally stacked but is too fat). So should i trot him the whole time to gain muscle or lope him to lose weight at the risk of losing muscle? over time i'm sure he would lose weight by trotting but i don't have long... Is he right on some degree or is it crap information? what else can i do for building muscle, i already do alot on hillsides....