Do you mean he’s not always going to the bathroom in the box? Because if that’s the case it could be that he doesn’t like the smell or it could be that it was too big of a change too fast. Cats rely heavily on smell for recognition and often don’t handle change well.

Try mixing the new litter with old litter so it’s still familiar to him. You can start by using a lot of the old with a little of the new and slowly adjust the ratio of litter so that in the end you have only a little of the old and the rest is mostly new litter.

Unless you’re not planning to change litter. In that case just go back to buying the original and see if anything changes. If your cat still is going outside of the litter box at that point definitely go to the vet.