Let f, s, m represent my father's age, my son's age, and my age, respectively.
.. f = 5s . . . . . . . . . . . . . . my father's age is currently 5 times my son's current age
.. s +(f -m) = 8 +m . . . . . when I'm as old as my father, my son will be 8 years older than I am now
.. f +m = 100 . . . . . . . . . the sum of my age and my father's is 100
These can be rewritten into a matrix form that makes solution using a calculator easier.
.. f -5s +0m = 0
.. f +s -2m = 8
.. f +0s +m = 100
A calculator shows the solution to be f=65, s=13, m=35.
My son is now 13 years old.