7yo struggling with positive outlook

I for context in the last year my son has lost a grandmother 2 cats and 2 dogs. Most the time he is a happy go lucky kid but there are times he can get very sad understandable. When asked he says he misses them. I try to be sympathetic of feelings but try to get him to focus on the happy times and to know that none of them would want him to be sad. This is what worked for me when I lost my mother at a young age.

He explains how that’s hard and just has a negative outlook on anything I try to do to help.

Just wondering if anyone has suggestions or books to share we love the emotions books.