This is a game cheat but it should work to fix your prob.
1. Control + shift + c
2. Type "testingcheatsenabled true"
3. Escape
4. shift click on your prob Sim
5. Click edit Sim
6. When page loads just move the bar for the desired Sim weight and any other things you wish to change on your Sim at the moment. Traits, hair color, clothes etc...
7. Click the check to exit CAS (create a Sim) mode
8. save game
You Now have your new & improved fixed Sim to Enjoy!
If you are using a cheat to make his hunger be ok? That happens with me it is because the game thinks he ate a little too much to fix it do Testingcheatsenabled true the do a shift and hold on the fatty and click on edit sim and lower his weight and then save