The restriction for calling someone Abu-l Qasim was only at the time of Allah's Messenger (blessings of Allah be upon him and peace).
Praise be to Allah.
Using the names Qaasim and Muhammad, and the nicknames Abu’l-Qaasim, Abu ‘Eesa and other permissible names and nicknames, is allowed and there is nothing wrong with it, after the death of the Prophet (peace and blessings of Allaah be upon him). The prohibition of using his name and his nickname together applied only during his lifetime (peace and blessings of Allaah be upon him). The reason why the prohibition was narrated indicates the reason why this was restricted to his lifetime (peace and blessings of Allaah be upon him). Hence you find scholars of earlier and later times called Muhammad and using the nickname Abu’l-Qaasim, without anyone denouncing them. Shaykh ‘Abd al-Kareem al-Khudayr
Shaykh Bakr Abu Zayd said in his book Mu’jam al-Manaafi al-Lafziyyah:
It was reported that Jaabir (may Allaah be pleased with him) said: “A son was born to a man among us, and he named him Qaasim. We said, ‘We will definitely not give you the nickname of Abu’l-Qaasim.’ He told the Prophet (peace and blessings of Allaah be upon him) about that, and he said, ‘Call your son ‘Abd al-Rahmaan.’ (narrated by al-Bukhaari in his Saheeh).”