You seem to be a reliable ev trainer, BUT, Triggzz (sorry if mis-spelled) trust you well, as is shown, muffin however, doesn't have that trust that you to have with you yet, and it is all down to personal choice, which we must all respect.
I'm sure that if you stop VMing him and give him some room, and act nice to/helpful to him, trade some pokes together and get to know eachother a little more, he will let you ev train for him, but these things are personal choices, and require time in some instances.