You have said that Phyllis married in 1959 and divorced 5 years later, because of a son from another man.
So the son born in the 1960's.
Unfortunately rootschat has a no living persons policy, so you need to go to another site if you want to try and trace him.
If he is remembered, then do they know his name(only, do not put it on here, but you can on other websites)?
From the death certificate, you have another daughterr's name - have you tried to trace her?