That your stbx is fixated on the weight thing is another reflection of himself. He is unhappy on the inside so he will focus on outside things that he thinks he can change.
You are doing the right thing not mentioning their weight gain to your kids.
The obsession and focus on weight is such a waste of time and energy. Encourage your kids as you do to stay fit and healthy with swimming and riding.
---- M 39 H 35 D5,D4 M 4 T 9 ILYBNILWY 5/18/11 Left 7/11/11 Divorced 12/1/13