No fix that I'm aware of. Go ahead back to your previous save.
I had the invisible Sim go to the dresser and change clothes, and that worked for him. If he bugs out again I'll go back to save 16, which was the first one before I had the error. So, for anyone else that gets the invisible sim bug, I'll report back on whether he stays visible now.
Edit: I narrowed it down and it's either linked to the Wish Enhancing Serum, the Lifetime Wish (curing 12 sims for witches) or the Age Freeze potion he used. His LTW has disappeared in the sim bar, but he still has the points for it, so it's a viable fix.