According to many definitions, a disability is an impairment that may be cognitive, developmental, intellectual, mental, physical, sensory, or some combination of these. Other definitions describe disability as the societal disadvantage arising from such impairments. Disability substantially affects a person's life activities and may be present from birth or occur during a person's lifetime.
The father or paternal grand father is the guardian of a disabled adult.
There is a supplication from Imam Zayn al-'Abidin (a) related to ill health.
His Supplication when Sick or Visited by Distress or an Affliction
1- O God, to Thee belongs praise
for the good health of my body
which lets me move about,
and to Thee belongs praise,
for the ailments
which Thou causest to arise in my flesh!
2- For I know not, my God,
which of the two states deserves more my thanking Thee
and which of the two times is more worthy for my praise of Thee:
3- the time of health,
within which Thou makest me delight in the agreeable things of Thy provision,
through which Thou givest me the joy to seek
the means to Thy good pleasure and bounty,
and by which Thou strengthenest me
for the acts of obedience
which Thou hast given me success to accomplish;
4- or the time of illness
through which Thou puttest me to the test
and bestowest upon me favours:
lightening of the offenses
that weigh down my back,
purification of the evil deeds
into which I have plunged,
incitement to reach for repentance,
reminder of the erasure of misdeeds through ancient favour;
5- and, through all that, what the two writers write for me:
which no heart had thought,
no tongue had uttered,
and no limb had undertaken,
rather, as Thy bestowal of bounty upon me
and the beneficence of Thy benefaction toward me.1
6- O God,
bless Muhammad and his Household,
make me love what Thou hast approved for me,
make easy for me what Thou hast sent down upon me,
purify me of the defilement of what I have sent ahead,
erase the evil of what I have done beforehand,
let me find the sweetness of well-being,
let me taste the coolness of safety,
and appoint for me
a way out from my illness to Thy pardon,
transformation of my infirmity into Thy forbearance,
escape from my distress to Thy refreshment,
and safety from this hardship in Thy relief!
7- Thou art gratuitously bountiful in beneficence,
ever gracious in kindness,
the Generous, the Giver,
Possessor of majesty and munificence!
There are advantages and disadvantages to marrying closely (like a first cousin). Sometimes the advantages, such as having greater trust, might outweigh the disadvantages. To my understanding, occasional close marriages are not usually a problem with genetics, rather, problems tend to arise when people do this over a long span of generations. Also they have been common in many cultures not just among some Muslim cultures. Genetic problems can also arise in any marriage. However there is nothing in Islam recommending this particular type of marriage.
It would be best for such a person to find an occupation that is free of any kind of physically strenuous work. Maybe select an office job, or something that you would be good at, and also be able to earn an income from.
With prayers for your success.
His mother can continue nursing him as much as he needs nursing even when he becomes a Baaligh, because it is a medical requirement where there is no other option.