Advice on managing grief from pet loss with 4yo?

My son and our cat Bubbles were so close. She spent every waking moment with him, and was a tremendous presence in our house (that says a lot because we had two other cats as well). She died of hyperthyroidism when my son was 3.5, it’s been 8 months and he’s still grieving her. We talk about it openly with him, we grieve with him, validate his feelings, reassure him…but he’s still very sad. The other two kitties are still around but their relationships are nothing like his bond with Bubbles. Does anyone have advice on how to navigate this? We are also looking into counseling for him, I just want more suggestions on how to address it on the home front.