Assalamualaikum ustaz. Is it permissible for a mother to get married with her son acting as her wali?
Waalaikumussalam wrt. wbt.,
Alhamdulillah, praise and gratitude to Allah SWT for His countless blessings for us all. Praise and salutations to our beloved Prophet Muhammad PBUH, his family, companions, and all those who follow his footsteps until the Final day.
Wali or in Arabic al-Wali (الوَلِي) lexically means “near or close by”. It is stated as “walyyahu, walyan” which means near or in its proximity. And “wali al-Amr”, when one is dealing with him and “Tawalla al-Amr” which means he is appointed.
The terminology, al-Wilayah is used as the highest leader, qadi, army and others. There are various meanings for the word wali or wilayah, it also means a proxy or representative or guardian that is likened to the wali in someone’s lineage, proxy in inheritance division and others. 
Furthermore, the other meaning al-Wilayah is it is the antonym of al-A’dawah, which originally means love and closeness. While the meaning of al-A’dawah is hatred and distance. 
Whereas wali in marriage is a person who determines the validity of the solemnization (marriage). The solemnization is invalid when he is absent. A wali can be the father, a person who is bequeathed, a close family member (al-Qarib al-Ashib), a person who freed a slave, a ruler and a king. 
The presence of a wali is obligatory during the solemnization. A marriage of a female regardless of whether she is an adult or a child, widow or virgin must be officiated by her wali. A woman shouldn’t be married, give herself in marriage or marry off another woman, whether it is with the permission or not, even if there are ijab and qabul from the woman. This is based on a hadith from Abu Hurairah RA, Rasullullah PBUH said:
لاَ تُزَوِّجُ الْمَرْأَةُ الْمَرْأَةَ وَلاَ تُزَوِّجُ الْمَرْأَةُ نَفْسَهَا
“A woman may not give a woman in marriage, nor may she give herself in marriage,” 
The companions R.Anhum called a woman who gives herself in marriage as an immoral woman, whilst in another narration she is mentioned as an adulterer. 
According to the above question, a son isn’t permissible to be the wali for his mother. The author of al-Fiqh al-Manhaji stated: A son and grandson from a son isn’t permissible to be the wali in a marriage. Thus, as a son, he is impermissible to be the wali for his own mother, for they aren’t from the same nasab. The reason is they are both nasab to their own fathers. The son of an uncle is permissible to be the wali of his aunt if there isn’t any close wali to give his aunt in marriage. 
To conclude, in our opinion, a son will feel uncomfortable giving his mother to marry another person due to his relationship with his real father. This is a natural feeling present in him.
May Allah grants us all understanding and knowledge in religion and that we can implement them in our lives. Amin.
 See al-Mausu’ah al-Fiqhiyyah al-Kuwaitiyyah, 45/135
 See al-Furqan Baina Aulia’ al-Rahmah wa Aulia’ al-Syaitan, pg 9
 See al-Fiqh ‘ala al-Madzahib al-Arba’ah, 4/29
 Narrated by Ibn Majah (1882), al-Bazzar (10085) and al-Daruqudtni (3/227). This hadith is evaluated sahih according to the book Sahih Ibn Majah (1539)
 See al-Fiqh al-Manhaji, 4/61
 See al-Fiqh al-Manhaji, 4/63