Sifuu mentioned that she and Hassian were unpacking Taylin's old things. Maybe you can check around their house for clues on what's bothering Hassian?
Sifuu: An old star chart?
Sifuu: Oh, I remember this! Taylin and Hassian used to go stargazing all the time.
Sifuu: I never had much patience for it. But she used to pore over this book with him, teaching him all the stars. How to spot them.
Sifuu: I asked him once, after everything happened, if he wanted to watch the stars with me instead.
Sifuu: He said no.
Make Hassian a new telescope in order to cheer him up.
Sifuu: Great! Let me just take these... shape the scope... fix the glass...
Sifuu: Aaand done! If Hassian hates this then my name ain't Sifuu.
Sifuu: Go get 'em, tiger!
You made Hassian in a new telescope just like the one he used to watch stars with Taylin! Hopefully this will make him feel better about missing his momma.
It seems like Hassian still has more to say. Give him some time, and check back later.
Hassian's asked you to meet him tonight in his grove. Talk to him there during the hours of 10pm-3am.