You can only spot clean a Graco harness, not submerge it, so I'd call Graco and order a new harness, ASAP. Take the cover off, and machine wash it on gentle. They're hang dry, but I put my covers in the dryer on no heat (important because they will shrink) because it speeds up the process. Once you have the cover off, you'll be able to see where all it went into the little nooks and crannies and clean it out.

I hope he's feeling better!