Learn German: Mein Sohn ist männlich.

In German, adjectives only get endings when they come directly in front of a noun. Because männlich comes after the verb ist and just describes the subject, it stays in its basic, unchanged dictionary form.

Is männlich related to the word Mann (man)?

Yes! The suffix -lich is often added to nouns to create adjectives, similar to "-ly" or "-ish" in English. When adding this ending to short nouns with an "a", "o", or "u", the vowel usually gets an umlaut, which is why Mann becomes männlich.

Can I use männlich to talk about grammatical gender, like "masculine nouns"?

No, männlich is only used for biological sex, meaning male humans or animals. When talking about grammar, like a masculine noun, you use the word maskulin.