Is he flirting, or should I assume we're still just friends?

Last week, a man I've been casually friends with for several years invited me to do private tennis lessons with him. That's his side business, and I've always known that, but we chat in person at least once a week and he's never invited me to spend time with him one-on-one before.

He knows I've never played before and doesn't seem to advertise his business to grow, so there didn't seem to be a clear reason why he would single me out for it and it didn't seem like a "salesy" thing. He could have tried to convince me years ago if he just wanted more clients/income.

We also don't have each other's contact info, as we just know each other through church, but he didn't give me a business card or offer to exchange numbers. However, in the last few months, we have been having longer and more frequent conversations when we see each other in person. If I don't stop to talk to him, he'll sometimes come up anyway and seem to make up an excuse to talk to me or at least say goodbye if he can catch me on my way out. He even made up an excuse to take a photo of me, which we've never done before.

I'd like to bring it up again and do a lesson just to see where this is going, but I don't know if he is just finally feeling more comfortable with me as a friend or if he's gearing up to ask me out. (To be clear, I would absolutely go on a date with him. I just haven't been asked out in a long time, so I don't want to read into it and get my hopes up.)

What should I be expecting, and how else can I test the waters without risking our friendship?