Give him another set of clothes, only one at a time, and make sure they are a different color from whatever you gave him before. He should put those on and be fine.
Maybe he thinks it's bathtime. Except for "Heavy Rain", the characters in all video games have to bathe/shower in their undies.
Sims uses the huge pixel censorship. Removing it they are nvde. However you also see that they do not take down the pants then using the toilet. After an friend did something similar while drunk the patch came back on fast.