I take care of a 7 year old autistic child & he's had a cat and a dog that had to be put to sleep and we told him & his twin sister straight up and they seemed to understand. If anything I was more disturbed than they were. defy how attatched was your son to the cat? I know he's only 3 but if you explain to him a version of the truth I'm sure he'll get over it fast. Getting a new pet will almost always help.