what do you mean by "fixes"
No problem, the NSRA policy is one of providing a level playing field for all participants regardless of any disability. The procedure is quite straightforward:
From the NSRA Rules:
4 SHOOTERS WITH PHYSICAL HANDICAPS
4.1 A shooter who because of physical handicap cannot fire from one or more of the prescribed shooting positions outlined in these rules, or who must use special equipment when firing, may apply to the Referees Committee for permission to assume a special position or to use modified equipment, or both. The application will be in the
form of a written request from the person concerned to the Committee outlining in detail the reasons why the
special position must be assumed or the special equipment must be used. The application will be accompanied
by pictures of the shooter in the position he desires approved and if special equipment is required, the picture
will show how the equipment is used. The application and all pictures must be furnished in duplicate.
4.2 The Committee will review each application and may require additional or supplementary statements or
pictures. If approved a special authorisation certificate will be issued, subject to any restrictions that the
Committee may decide. Such certificates will have the relevant pictures attached. In the event of the
Committee having doubt as to whether the issue of a certificate would give undue advantage over other
shooters, the Committee will seek advice from other experts.
4.3 The grant of an authorisation certificate does not depend on the assessment of disability by an organisation for disabled people approved for that purpose by the NSRA. However, if the disability is of a type recognised in
the shooting rules of an approved organisation, the shooter will normally be authorised by that organisation
and required to shoot in accordance with the technical rules of the International Shooting Committee for the
Disabled of the International Paralympic Committee (ISCD).
A club instructor or coach would be able to advise and help with this.
Now by the sound of it your son would be fall into the SH2 class under ISCD Rules:
12.3.3 Specifications class SH 2:Rifle competitors who have measurable and /or visible permanent disability in upper limbs and therefore are unable to support the weight of a rifle for the purpose of the competitions with their upper limbs and require a shooting stand.
The stand referred to has a spring that allows movement but a the same time supports the weight of the rifle.
Now the issue with air pistol is that there is no SH2 category as it is judged that it is too difficult to establish whether at support stand might constitute unfair advantage.
Find a club with knowledge of disabled shooting and take it from there. He's probably left it a bit late for the 2012 Paralympics and might still be a bit too young on 2016. But in 2020 he will still only be 19 and have had 9 years to prepare - go for it!