Is copyediting good for an academic career?
The only benefit you'll get as a copyeditor is clear: money (assuming he offered to pay you).

The downsides are numerous: it takes a lot of time, it will not be considered a plus to your CV (it's a technical job, not a scientific one)… none of the benefits from being a journal editor apply to a copyeditor.

Also, I don't know exactly what you did when you say “I have recently been involved in managing a special issue”, but if you were guest editor, offering you a copyeditor job is clearly not showing appreciation for your job as a scientist.