I have never understand the not "remembering" to turn in homework; doesn't the teacher ask for it or your grandson see others doing it, etc.? I would talk to the teacher, get your grandson a "special" folder in a bright color to hand to the teacher each day or something like that. It is a multi-step process, getting/understanding the assignment, doing it, taking it with one to school and then turning it in at the "right" time. It is not at all intuitive and doesn't make much sense. I can see how after doing it, one can feel one is "done" and not think to do anything else with it; my part is done. . . Maybe some sort of reward system so he gets feedback when he gets home for having turned it in?

Get your husband in the act by making him part of a reward system. They will learn to enjoy/be proud of each other working together? Maybe start with a chart/with stars on it, something like that if your grandson likes doing art. Or get some colorful stickers with sports or whatever else it is your grandson likes? Maybe some he doesn't like versus some he does like so he has to put a sticker he doesn't particularly like on the chart when he forgets but a good one when he remembers.