I've never heard of tea causing kidney stones; cultures around the world drink tea, and have been drinking tea for millennia. I always thought it was a safe way to get plenty of water. I could be wrong, though; I haven't researched it.
I can offer my own anecdotal experience, for what it's worth. Several of my children (all ASD) almost exclusively drink black tea all day, no sugar added, and I've never had a problem with anyone getting kidney stones. My kids are all under 20 years old. (So it's possible it just takes longer for the stones to develop, maybe?)
Like timf, I think bribery can be a useful tool. It took awhile, but I was able to successfully expand my non-verbal son's diet by saying, "You can have [favorite food] if you have a bite of [different food] first. Just one bite." He refused the deal the first 20 times. But after he finally tried it, I kept it up for a month (or two, or three, or *twelve*), then the negotiation involved three bites, and then eventually, an entire serving. After a few years, he does things now like voluntarily devour half a rotisserie chicken. (He's an older teen, currently.)
Maybe the bribery worked, or maybe it was just a leap in development or simply a change in tastes. No one ever really knows.