Teenager has problems finishing homework. Any advise?

Patterns matter. Is this a new issue or a longstanding pattern? What is getting in the way of finishing homework? If this is new, talk with teen and set up clear expectations and consequences. If it's longstanding, then professional evaluation and help may be needed. Start with the school; they should have resources at no extra cost to you to look into the problem.
Distractions? Set up time periods for homework. Give him breaks every 20-30 minutes. Has this been a pattern from earlier years? He may have some learning problem (add?) may want to talk with your M.D.