Although my first reaction was that he is too old, I do agree with Ethel that perhaps if you just back off and let him have his little securities, they will pass on their own. If he is only doing it at home then really, it isn't a big deal. My twins are 2 1/2 and my older son is 5, the only time we use sippies is at bed times (nap and night) which then they only have water in them, which rarely is touched by morning so it is a security object. Perhaps if you limit it just to bed time and let him pick out some cool big boy cups, like some of the straw cups they sell with characters on them and encourage the use of those at meals. All of mine use open top cups at meals and during the day. My older son takes a straw cup or open top cup to bed, it sits on his night stand and he rarely drinks it.
I liked the suggestion to perhaps switch the type you are using. I would make sure it has the harder plastic spout so they can't nurse on it or carry it in their teeth. Taking the valves out will kind of ruin the sucking effect for them so that might make a difference also. I babysit for a 5 year old who started kindergarten and that is all he uses at home, even after almost a year with me he still has a very hard time with an open top cup at my house, and thus I would assume at school. If we go anywhere we bring sports bottles with ice water. I never serve milk or juice out of a sippy, even for my not quite 2 year old daycare girl, she only gets open cups with milk and juice at meals and does somewhat well with them but like her brother, only gets sippies at home so it is a tough battle!
I think the fact that he has overcome the need for it at your mom's house says a lot. Clearly there is a power struggle that he is choosing to have with you, thus perhaps letting it go and not making mention of it at all could take that struggle away. What if you make him totally responsible for the sippy: he has to fill it at night, put it in the dishwasher in the morning etc, then if it isn't done, he either doesnt get one if he doesnt fill it, and if he doesnt load it, then it goes away at which point they would slowly all go away but it is all in his hands as it was his responsibility?
M.