No news is good news. Not hearing from your son just means he's doing well.
Literal
Letter's not-having [topic-のは] good letter. Son [from-から] contact [subject-が] not-having [topic-のは], well doing because [assertion-よ].
Opens with the proverb 便りのないのは良い便り — a direct Japanese equivalent of 'no news is good news.' The second sentence applies the principle concretely: ~のは...からよ explains that the reason you haven't heard from your son is that he's doing fine. The final よ softens the reassurance with a warm tone. 息子さん with さん is how you refer to someone else's son.