I'm sorry for your loss
Was he ever taken to the vet? And was the current degu taken to the vet in case it's contagious? If not, do that asap.
Pet store degus aren't always the healthiest so if possible, purchase future degus from a breeder or adopt adults known to be healthy from a rescue.
What do you feed your degus?
You can (and should) introduce a new degu but only if the current one gets a clean bill of health. A baby will be easiest to introduce and there are a ton of great threads on here with peoples' experiences introducing degus