Question Summary:
What is the Islamic ruling about my son who has Down Syndrome and wants to marry?

Question Detail:

what is islamic ruling about my son 23 year old have a down syndrom he want to marrry the girl is agree what to do ?

Answer :

In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
A marriage with a handicapped person is very challenging to the handicapped person and the spouse. The guardians of the handicapped should exercise extreme precaution in making a proper decision.
If the girl is fully informed of your son’s medical condition and she decides to marry your son, the marriage will be valid if your son is mentally stable and he does not have any mental deficiencies in him.¹ 
And Allah Ta’āla Knows Best
Huzaifah Deedat
Student Darul Iftaa
Lusaka, Zambia
Checked and Approved by,
Mufti Ebrahim Desai.
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1. https://en.wikipedia.org/wiki/Down_syndrome
اللباب في شرح الكتاب (ص: 254)
(وينعقد نكاح) المرأة (الحرة البالغة العاقلة برضاها) فقط، سواء باشرته بنفسها أو وكلت غيرها (وإن لم يعقد عليها ولي) ولم يأذن به (عند أبي حنيفة: بكراً كانت أو ثيباً)، لتصرفها في خالص حقها وهي من أهله، ولهذا كان لها التصرف في المال