Firstly... thank you thank you thank you for being such a good cat parent!
Cats are super finicky; mine would only drink from a wide casserole dish, because his whiskers brush against the sides of all other bowls/fountains. Some cats don't like stainless steel bowls (possibly the way the water edge blends with the steel side?); figures since it's healthier for them. Is the shape of the bowl itself or the way water flows different? Does it have a different filter compared to the previous fountain?
If none of those are the case, you could try running the new fountain for 24hrs to see if the water flow helps wash away any lingering funky factory smells.
You could also try a more gradual transition; kitties get fussier as they get older, even if he is still technically young. Perhaps turn the old fountain off, but leave it beside the new fountain (both with water) and see what he goes to. Then eventually keep the water out of the old fountain, but leave it beside the new one so aid in comfort and guidance.
Also, keeping the water away from their food is recommended.
Over all, I wouldn't let your kitty go without water for more than 24hrs. If he still won't shift over to the new bowl, you can always try switching the old fountain component out for the new one, or buy a fountain component replacement online.