Look dude, we understand that you are hurt because she left you for another. And even worse she left you for a "fat flabby girl". But your attitude stinks. Your kid WILL have 2 mothers and a father. That is a fact. If you go around him calling them "cunts" and all sorts of stuff, you will fuck him up, because whether you like it or not he will have affection for them both and there is nothing worse for a kid than having to chose between parents.
So for the kids sake, man up and stop being a jerk. There are loads of decisions that you and your former partner must be able to cooperate around, so you must find a way to be civilized around her.
And back to your question, the kid should not have a phone with him to kindergarten. Not only does it disrupt the kindergarten but it will also get destroyed or lost in a week. Even if he is a little kid he must be able to feel he has his own space, not being constantly on guard because daddy might call. Give him a cheap android tablet that he can have around the house. Then he can be in his room and skype you, without you risking seeing naked people.