Zylkene (milk protein) prescribed when cat has calcium oxalate crystals?

Okay so as the title says, my cat (neutered male, about 10 years old, 17 lbs) has a history of calcium oxalate crystals. After a recent flare up, the vet noticed he continues to have blood in his urine even though there are no longer crystals, and he is also peeing more often than normal and in smaller amounts at once.

They can’t find any cause so they think it’s stress, and prescribed Zylkene and Cerenia

Zylkene is casein-based, so I’m worried about this given the calcium oxalate crystals. I asked the vet, and she didn’t know whether that would be an issue or not. She said we could do a urinalysis in a couple weeks to check, but he’s had two blockages and I’m not sure I want to take that risk.

Does anyone else have an opinion on this?