When my son, who is 83lbs rides in the passenger seat, the airbag OFF light comes on. This behavior does not seem to be consistent. Does seat position have anything to do with this? Is his weight too low to trigger the sensor? Or is it something to refer to the dealer?

This query is for a 2013 RX350 with about 1,700 miles on it.

 

This is discussed starting on page 170 of your Owner's Manual:

http://drivers.lexus.com/t3Portal/do.../sec_01-08.pdf

Believe there is a concern about airbag deployments for when a small(ish) child is in the front passenger seat. There is a whole section on page 174 which discusses about posture and seat position affecting whether this indicator is on or off. Can happen to adults as well.

Sounds like the system is working as designed.

 

Yep, my mother-in-law is a very tiny 85 pound woman and when riding in the passenger seat gets the light the same as your son.