My Mom Put Sugar In Our Water Gradually Less And Less Until We Got Used To Drinking Water Alone. Can't Speak For Myself But My Little Brother Never Noticed The Difference.
Throw all the bottles out, hes going to cry but oh well deal with it. Dont give in to his tantrums. If he goes to grandma to get a yes take him to his room and stay in there and tell him no. Your the mom and the boss so he needs to listen to you. If she still tries to give him stuff you don't want him to have simply take it away. Of course hes going to cry but the more you give in the more hes going to do. When you give him juice dilute it, half juice half water even if he doesn't like it you tell him it thats or nothing. Hope this helps.
Hes hella old to be drinking outa of a bottle . I had that problem with my niece at the age of 3 . I hid all her bottles and gave her food instead of milk and juice she was drinking out of a sippy cup in a week in a half before my brother and hes wife came back from there honey moon . Just stand ur ground be very prepared for him to cry but remember u are the adult not him . U say what gos with ur child not ur mother in law . And i would also sit her dwn and tell her that she has to help u cus ur sons teeth and speech is in jeporady if she countinues with the bottle givin . I hope all gos well with ur mother in law i kno how they can be with the whole spoiling ur kids but remember u are the mother she is the grandmother ur word is always first and she cant fight or be angry at u for that .