@Bob: The article doesn't add much to the sentences (unless you mean to emphasize he's the most important or the only son of someone), so it's unnecessary. In the first sentence, though, the order of the words makes it sound awkward, so it's better not to use it.
- Sin importar hijo de qui
én* seas, no me faltes al/el** respeto.
- Seas hijo de quien seas, no me faltes al/el respeto.
- No me importa de qui
én seas hijo, no me faltes al/el respeto.
*The use of the written accent is different, because "
quien" (relative pronoun) can be replaced by "la persona que" and "
quién" (interrogative pronoun) can be replaced by "qué persona".
** In Mexico, it's more common to hear "faltar al respeto", but there are many other regions where "faltar el respeto" is preferred.