HOW DO I EXPLAIN IT TO HIM?

We were at Fry's the other day and parked at the end by the garden center. This is the end close to the Vet where we had to put Ricky down last year. Everytime I think AJ has forgotten or understands he says something like he did this day. When we were leaving he told me he wanted to go to the vet and get Ricky back. It was all I could do to hold back the tears right there in the parking lot. I guess because I still feel guilty for putting him to sleep, I wish we could have kept him comfortable longer and kept him longer. We stopped there and I explained, again, that Ricky wasn't there anymore. His body was buried and his spirit is in heaven and we can see him when we get there. I reminded him that Ricky isn't hurting anymore and is playing in water and running around and keeping Gramma and Papa company. He said he wants to go to heaven with Ricky. I didn't know what to say so I just said that someday he will, when Heavenly Father is ready for him to. Even now just thinking about it I'm trying not to cry because I don't want to get them going again. He was such a great dog, he deserved better than to suffer for so long but I still miss him. I don't know what to say to AJ to help him understand better.