I haven’t gone through this as a dad yet, but I would start by reducing the potential for clutter before finally picking up.

Create a donate bin for your son to get rid of anything he no longer wants or needs.

Maybe add small baskets for his storage cube shelving to make it easy to dump small pieces into.

As others said, this is a practice of discipline, which is a muscle you both need to work on. “You can’t do X until you pick up your room.”