My kids were 12 (she was one month shy of 13) and 15 (just had his birthday 5 days before). I have struggled the entire time knowing if the behaviors are grief or teen. I think it is a fine tightrope we walk in raising these kids alone with grief. It will get a little easier to differentiate the older they get. My daughter cries all the time that she just wants to be normal and have a normal family. And when pressed, it is mostly teen angst. Guess what, kiddo?? THIS IS NORMAL NOW.
I would certainly listen to and sympathize, even empathize that you miss her, too. "But she's gone, lets make Mom proud by showing her that you can rely on me, too". I would hesitate to say anything like we have each other, or something of the sort. Both of my kids seem to think my emotional well being is their responsibility. I don't think you want to foster that.
Anyway... Your loss is still new, fresh, painful for everyone. Be gentle and allow the extra dramatic responses, but help him pick himself up and tell him that you are doing it because mommy taught you so well how to do it.