If feasible, help your son/daughter use the free, online survey at https://www.viacharacter.org/www/Character-Strengths-Survey to get their personal profile ranking of the 24 character strengths. Their signature strength(s) should be in the top 5–7. The link to the surveys (youth and adult) includes a guide for using the youth survey with youth with intellectual and developmental disabilities.
You may already know the character strengths your son/daughter uses, and if you notice which strengths come naturally to them, are easily used in different situations and make activities more engaging, these are probably their signature strengths.
This link provides a snapshot look at the 24 character strengths.http://www.viacharacter.org/www/Portals/0/Graphic2014.pdf