How do I get him to go?

My Son will be three June 11 and im trying to potty train him… he’s very stubborn but shows all t he signs that he is ready and interested. His father who im no longer with has him every other week.He say’s that I need to get him to go in the toilet because he is trained at his house. But im not sure I believe him because you would think if he is potty trained there and I show him the potty he would be trained at my house as well… My son’s father say’s it’s ridiculous that he’s not going at my house and that he’s starting not to go at his house as if it’s all my fault. Just because he want’s him to go to school when he turns 3 years old… I can’t force my little one to go as I’ve told his father but he’s not hearing it…He want’s me to keep my son for a couple of month’s just to get him potty trained which I don’t mind because I’d rather see my son everyday but it makes no sense and I don’t understand why my son’s father has to get mad about it. And I just can’t get my little one to go. I constantly ask my son if he has to go every sec. and he say’s “No mommy I don’t got to go.” So I ask a little later then bring him to the potty and tell him to try to pee and aim for the sticker.. He knows how to pull everything off and aim but just doesn’t go and want’s to go back to playing…and so I’ll ask later again “Do you got to go potty now?” and he say’s “No” but when I check him He is pee or poop… And I’ll tell him he was supposed to tell me so I could bring him to the potty. I just don’t know what to do and my son’s father keep’s asking if im even trying to potty train him so he’s bugging me and stressing me about it… What can I do?