Unfortunately Mad your son will want a pet he can interact with, a goldfish hardly fulfills this requirement. Dogs are by far mans best friend but can have muddy paws, smell when wet and require great input. Cats can sometimes scratch furniture but on the whole tend to look after themselves and are clean. Most other pets require cleaning at least once a week or they tend to smell and can become unhealthy. One solution may be to enroll him has a volunteer at a local animal sanctuary other than that you have to accept any pet comes with its own problems.