Mother/ binky help!!!

I need advice on how to handle my mother when it comes to my son using a binky! My mother watches my son, who is almost 2, monday through Thursday while I'm at work which i am very grateful for! But at around a year and 3 months, we cut my son off the binky cold turkey and he's done absolutely amazing without it and only wants one if he sees it but my i keep catching my mother giving it to him when she has him. I've asked her many times to please stop giving it to him but i still catch him with it in photos she takes off him. Im currently almost 7 months pregnant and really want to kick this binky thing before the baby arrives so he doesn't constantly throw a screaming fit because the baby has one and he can't and i don't want him going back to the binky at this age. Really looking for advice on how to either convince my mother to stop giving it to him or even tricks or other options of things to give him for when he does see a binky so that he wont want it. I have a very good relationship with my mother so being mean and just saying that "im the mother and what i say goes" isnt an option and i cant beat the price of babysitting if i were to send the two boys to daycare instead of her watching them, plus she loves spending the time with him/them and i love that he gets to spend time with someone he knows rather than a stranger at a daycare center so im really looking for a way to fix this that works for all three of us! Thanks so much in advance!