If the pooling of tips is company policy you have an employee who is violating policy (call it being dishonest) and is subject to management action, and you should report him. If the pooling is a voluntary agreement between employees, you're stuck with a coworker who's isn't going to participate.
If you report him you should feel as if you are doing the right thing. Company policies are to promote the best interests of both the company and the employees and this violation is undermining morale. It's only a matter of time before this translates into problems with customers.
Do the right thing and don't look back.
