Tell him you have now found a new personal trainer and see what reply you get from him.... if he sees that his mischief backfired thats when you can say gotcha back bit risky though
Fart spray - work out an exercise you can spray it his way every time you bend or crunch. Tell him you have a gippy tummy. See how well he can keep it together
Pull in a favour from a gym buddy to pretend to be your new P.T almost like you have been caught cheating then get the other ladies to meet you there and agree to swap trainers too. Xx
Buy about 4 fat suits all in different sizes,or the step by step pregnancy bumps you can get turn up every week in one then when you're wearing the heaviest one point out to the pt that using his sessions your actually putting on weight and your going to be leaving a review on his fb page about how unsatisfied you are