I first would like to offer my condolences, RIP Charlie
I've only had 1 situation where I've had to deal with a grieving pet, we had 2 labs that were 2 years apart in age. We had to have the older of the 2 put down and the other was so depressed. I think she knew he wasn't coming back because she didn't seem to look for him, but she didn't want to play or anything. She acted that way for a long time, we did get another puppy after a little bit and it took her a while to even get near the new one, honestly she never really got to be very close and we ended up losing her less then a year after him.