There is not a big difference between "Segare（悴）" to "Musuko（息子）". Principally they do mean "(my)son" or "(my)boy". but sometimes could suggest "penis".

Segare:倅／悴 1.modest expression of Musuko when it used against one's own son. 2.casual expression of Musuko when it used against other family's son.

Okayy! mina san Arigatouuuu!! <3<3

Both mean 'son'. Maybe the former sounds a bit literary. Musuko can be used to refer to either your own son or someone else's son, while segare can be used to refer to your own son but cannot be used to refer to someone else's son: Both うちのせがれ and うちのむすこ are fine, but you say ～～さんの息子(さん), not ～～さんのせがれ(さん).