How would you handle problem with child constantly stealing, lying

Dishonest behavior, the mouthiness i can handle but this one keeps stealing
from my youngest son who is newly disabled and he says oh he doesn’t know
anyway.

My son had some special things from around his transplant time and
they are all gone.  Just about every week i catch my other son stealing from
one of the other kids here, he brings other peoples thing home from school i
collect them in bag and return to school. he no longer has a backpack
because he hid the other kids things in it to take to school to sell or give
away or trade