As a first step, I would contact his teacher(s) and possibly the school counselor. Discuss the problem with them and develop a cooperative strategy for addressing the problem. You and his teacher can put into place a simple strategy of identifying his assignments, checking for completion the following day, and rewarding successful homework completion.
A good book to share with your son is How To Do Homework Without Throwing Up by Trevor Romain and Elizabeth Verdick. The book is aimed at students in grades three through six, and includes tips on setting up a homework schedule, how to get help with homework, etc. Another book is The Berenstain Bears and the Homework Hassle. You may want to read How to Help Your Child With Homework by Marguerite Radencich and Pamela Espeland. This book offers tips for halting homework battles with your child.
You are not alone. Homework battles are all-too-common between parents and children, whether the kids are gifted or not.