One of my Sims is broken

@LeaLink23  Start by deleting the five cache files in your Sims 3 folder in Documents\Electronic Arts.  These files should be deleted regularly anyway, and any time you have problems with the game; while troubleshooting, delete them every time you quit.  For reference, they are:

• CASPartCache.package
• compositorCache.package
• scriptCache.package
• simCompositorCache.package
• socialCache.package

Load your save, go to Edit Town, evict the sim, and put him back in the house.  You can evict the entire household, or just split the one sim off and merge him back in.  Any sim evicted like this will keep their job and relationships but lose their promised wishes and opportunities.  Use "save as" to rename the save (so you have a backup), quit, delete the cache files, reload, and see whether the sim is still broken.

If he is, you can evict the sim and put him in a different house, quit and reload, merge him back in, quit and reload, and test.  Again, use "save as" so you don't overwrite your backup save, and delete the cache files every time you quit.

 @puzzlezaddict  I tried to evict the household but as soon as I do it, the game gets stuck on a 'processing' loading screen endlessly.

@LeaLink23  Try switching to playing a different household temporarily, and evict the sims while the other household is active.  After you switch households, "save as," quit, and delete the cache files before reloading.  Do the same after you evict and replace the sims, and again after you switch back to playing them.

If this doesn't work, would you be interested in trying mods?  There is one that might help, although it's not guaranteed.  At this point, your sim may be too corrupt to salvage, although it's always worth trying.