Ah, no, it’s not a good idea to file down a walrein’s tusks, I’m afraid. This could damage them and lead to long-term pain for your partner. Instead, you should always let your walrein maintain the length of his tusks on his own by supplying him with sturdy tree trunks, bones, or specially made posts for pokémon with continually growing teeth. The only time you should touch a walrein’s tusks is to brush them or to examine them for any signs of disease, particularly around the gum area.
When it comes to balls, though, you might want to consider a sturdier ball. While it’s true that volleyballs are no replacement, they are designed to take more hits than a beach ball and can be painted to resemble one using nontoxic paint. If your walrein can tell the difference, though, the alternative is to make or buy tusk caps made of food grade/nontoxic plastic to temporarily “dull” the sharpness of his tusks. If you go with this route, don’t forget to remove these caps when he’s done playing with his beach balls or at least when it’s time to eat and train.
Best of luck!