P.
We just put our cat down in Jan. and my 4yr old was very attatched to her. Being that your son is younger it may be easier to get through. I told him that Magic is in heaven playing with all the other dogs and cats and that my grandma and grandpa are taking care of her now (which he seemed comfortable knowing that someone was watching her) and she is not sick anymore. He knew that she was sick and that the doctor could not "fix" her anymore. Just be very careful using the word sick. he understood it. All I said is that she is much happier up in heaven. Every now and then he gets sad and tells me how much he misses her and we talk about it and he says but she's not sick so I am happy about that. Kids get it, he may suprise you. Good luck. Just talk about it and he'll get through it.
Good luck and hang in there and tell Gramma to hang in there also. It's a hard thing for anyone to go through