Cosmo is at Meowy's Mporium. I think he has a lot of information about Montameeuws.
Maybe he knows something about the stuff you are investigating. Try showing him that stuff. ?
Show him the seeds. You can now access La Boule de Poils.
You got access to La Boule de Poils. Neat.
Still, Cosmo is not opening up. You need something to press him with. You will get there, eventually. Examine other leads.
So, you visited La Boule de Poils? And helped Tybalt?
Look at the street outside the Mporium.
Take the poster off the wall.
Go talk to Cosmo. He should open up now that the symbol is visible.