Due to a dual language immersion program my daughter is in, she's been in the higher grade of a combo class every year from 2nd to 5th grade.
I don't think it's ideal, but if he has a great teacher, it will be fine. They have a lot of tools for covering the material for both grades, and the students learn to work well on their own and with each other (while the teacher is working with another group.) A good skill to have. We only had one year when I felt the teacher was not effective. She had trouble staying organized herself and keeping the students focused. Several students dropped out of the program that year, but we stuck with it, and have been back on track since then.
As a student in the younger grade, he'll have the advantage of being exposed to higher grade material (without being responsible for mastering it).
We don't spend extra time with homework, although I do tend to have her spend less time with the material she's mastered, and more with things for which she need more practice.