Kulayni narrated a long hadih in his book “al-Kafi” (1/503-504):
“May Allah take my soul in service for your cause, now you may if you would so wish.” “. He then said to his guards to lead him to through the backside so that he (Al-Muwaffaq) would not see him. He stood up and my father also stood up and embraced him and he left. I asked the guards of my father and his servants, “Woe is you, who was the man whom you addressed by his Kunya before my father and my father treated him as he ddid?” They said, “This ‘Alawid man is called al-Hassan ibn Ali, know as ibn al-Rida .” This surprised me even more. The whole day I thought anxiously about his affairs and the behaviors of my father and I did not see him until it was night. He would usually say his ‘Isha (late evening) prayer and would study the things that he would need of the issue to present before the Sultan. When prayed and began to study I went and sat before him while he was alone. He asked, “Oahmad, do you need something?” I said, “Yes, my father. If you would allow me I would like to ask.” He said, “You have permission, my son, say whatever you would like.” I said, “Father, who was the man you met this morning and behaved with him the way you did towards him with so much glorification, greatness and reverence with such expressions as, ‘May Allah take my soul in service for your cause’?” He said, “My son, he was the Imam ((Leadership with Divine Authority) of the al-Rafida (those who refuse to accept the leadership people other than (Ali and his infallible descendants). He is al-Hassan ibn Ali, known as ibn al-Rida. He remained quite for a while and then said, “My son if the Imamat leadership would be removed from the Abbassid Caliphs no one from the clan of banu Hashim would deserve to become the Imam except this man. He deserves it because of his distinctions, piety, guidance, safety (from mistakes), his chastity, his worship, his beautiful moral characteristics and perfection. Had you had a chance to see his father you would have seen a man of great intelligence, nobility and excellence.” This increased my anxiety, thinking and anger towards my father and I thought his dealing and behavior towards him and his words for him were excessive
Thereafter, there was nothing more important to me than to ask more about him and investigate his affairs. Whoever of the members of Hashite clan or the guards, the clerks, the judges, the scholars of the law (Fuqha’) and other people that I would ask I would find him being glorified, mentioned with greatness and high position. I would hear very beautiful words about him and see him given preference over all members of Hashimite clan, their elders. The feeling of greatness for him increased in my heart because I would not find anyone of his friends or foe that would not speak good of him and praise him.” A certain person from the al-Asha‘ari beliefs present asked him, “What is the news about his brother, Ja‘far, O abu Bakr? (1)“
He said, “Who is Ja‘far that you would ask about him or compare him with al-Hassan (a.s.). Ja‘far was an evil doer in public, polluted with sins, an addict drukered, the lowest of all man and the most humiliating of his own self among men. He was worthless and little to his own self.
In the footnote you can see shia muhaqeq said about Jafar: “HE WAS FAMOUS FOR LIE”.
And the only reason of this attitude of shias toward that man, that he rejected existence of his brother’s son.