He assumed Ihram while wearing Underpants due to Ignorance
Question :
A year ago I went for 'Umrah and I was ignorant of some of the conditions. When I assumed Ihram from the Miqat, I wore short pants under my Ihram clothes, as I was ignorant concerning this matter. After I returned, some people informed me that this was not permissible. I performed 'Umrah again this year when I learned that it is not permissible to wear (Al-Makhit) seamed garments. Is there any sin on me for that?
Answer:
You do not have to perform a sacrifice as you were ignorant of the ruling, and ignorance is an acceptable excuse for doing this prohibited act. The sacrifice for atonement is only obligatory upon whoever does this knowingly and intentionally. You are not required to repeat the 'Umrah, as you did not do what corrupts it. Thus, the second 'Umrah you performed is considered a voluntary 'Umrah.
Source:
Ash-Shaykh Ibn Jibreen
Fatawa Islamiyah, Vol. 4 Pages 141-142