Is it considered unfair to the wife now, after she was the one suffering from difficulty conceiving and the costs of IVF due to his sperm issues which he did not bother to treat previously, for the husband to go for treatment now?

Do not worry about whether your husband is unjust or not for abandoning treatment, as treatment is not obligatory according to the majority of scholars. His pursuit of treatment now is permissible and contains good for both of you. A person may refrain from treatment for one reason and then accept it for another, so do not let Satan spoil your lives with this matter. Praise Allah for your offspring and contentment, and his marriage or healing will not harm you; rather, wish him well.