Can you tell him how you feel? Maybe he has different reasons than you think for not posting about his own son.

The tone of his online journal is very jokey and flippant and this may be the persona he wishes to adopt online. Could it be he is much more serious and has deeper feelings about being a father and his own son and just doesn't see it as fodder for his jokey online blog?

added later: sorry, I missed the part that you two are no longer together. I though he was your boyfriend/husband. If possible, I would advise not reading his online journal to avoid future hurt and misunderstanding.