I'm on the side of 'if it matters to the child'. If your son didn't show any interest, I wouldn't bring it up. Do what feels good for you. Some kids do have a relationship with the fish. We had a tank when I was about eight and when some of the fish died, I'm not sure I even caught on. We weren't 'close', the fish and I.
However, my neighbor's daughter's fish (who she fed each day and adored) died, and the family had a small burial. She was only 5 or so, but it mattered to her. Let your child's past interest guide you in this.
And if you need a book, "Lifetimes: The Beautiful Way to Explain Life and Death to Children" by Bryan Mellonie is a very solid, good book. It is well-loved at our home.