There are many sunni scholars who confirm Mahdi (as) is Imam Hasan Al-Askari (as) son. For example the great Sunni scholar al-'Arabi.
واعلموا أنه لابد من خروج المهدي (عليه السلام) لكن لا يخرج حتى تمتلئ الأرض جوراً وظلماً فيملؤها قسطاً وعدلاً ولو لم يكن من الدنيا إلا يوم واحد طوّل الله تعالى ذلك اليوم حتى يلي ذلك الخليفة وهو من عترة رسول الله صلى الله عليه وسلم من ولد فاطمة رضي الله عنها، جده الحسين بن علي بن أبي طالب ووالده حسن العسكري ابن الإمام علي النقي ـ بالنون ـ ابن محمد التقي ـ بالتاء ـ ابن الإمام علي الرضا ابن الإمام موسى الكاظم ابن الإمام جعفر الصادق ابن الإمام محمد الباقر ابن الإمام زين العابدين علي بن الإمام الحسين ابن الإمام علي بن أبي طالب رضي الله عنه
Sh Muhyiddin Ibn al-'Arabi, al-Futuhat al-Makkiyah, chapter 366, quoted by Abdul-Wahab al-Sha'rani, in Yawaqit wa Jawahir vol 2 and also Sh Mahdi Faqih al-Imani mentioned this in Al-Mahdi 'inda Ahl al-Sunnah, Vol 1, page 410.
"great Sunni scholar". Ibn al-'Arabi was not a "great Sunni scholar". He was an extremist Sufi who came up with Wahdat al-Wujud
‘Aqidah Ibn ‘Arabi wa Hayatuhu by Taqiy al-Deen al-Faasi, p. 21-23)
(The author) mentioned the response of some of the scholars:
Al-Qaadi Badr al-Deen ibn Jamaa’ah said:
The passages quoted, and other similar parts of this book, are bid’ah and misguidance, evil and ignorance. The religiously-committed Muslim would not pay any heed to them or bother to read the book to find out more.
Khateeb al-Qal’ah Shaykh Shams al-Deen Muhammad ibn Yoosuf al-Jazari al-Shaafa’i said:
Praise be to Allaah. His comment about Adam being called insaan is anthropomorphism [likening Allaah to His creation] and is a lie and falsehood. His belief that the idol-worship of the people of Nooh was valid is kufr. Anyone who says such a thing cannot be approved of. His comment that ‘Al-Haqq which is transcendent is the physical creation which you can see’ is false and contradictory, and it is also kufr. His comment that the people of Hood had reached a true state of closeness (to Allaah) is a lie against Allaah, and by saying this, he has rejected what Allaah said about them. His remark that they were no longer remote and that Hell became a blessing and a joy for them is a lie and a rejection of everything that was revealed to the Prophets; the truth of the matter is what Allaah said about that, that they (the people of Hood) will abide in the torment forever.
Ibid, 21-23)
Shaykh al-Islam ibn Taymiyyah said:
When the faqeeh Abu Muhammad ibn ‘Abd al-Salaam came to Cairo and they asked him about Ibn al-‘Arabi, he said:
He is a vile and evil shaykh who says that the world is eternal and does not see anything haram in any sexual relationship.
Ibid, p. 25
Ibn Hajar said:
Some confusing words of Ibn ‘Arabi were mentioned to our master Shaykh al-Islam Siraaj al-Deen al-Balqeeni, and he was asked about Ibn ‘Arabi. Our Shaykh al-Balqeeni said: he is a kafir.
Ibid, p. 39
Ibn Khaldoon said:
Among these Sufis are: Ibn ‘Arabi, Ibn Saba’een, Ibn Barrajaan and their followers who follow their path and their religion. They have many books in circulation that are filled with blatant kufr and repugnant bid’ahs, trying to interpret clear texts in very far-fetched and repugnant ways, such that the reader is astounded that anyone could attribute such things to Islam.
Ibid, p. 41
You can find many more Fatawa on him here
http://islamqa.info/en/7691