c'est un charmant jeune homme

Il/elle est is (almost) never followed by an article-noun (it is French , so there are exceptions). 

‘C’est un charmant jeune homme’ is grammatically correct. ‘Il est un charmant jeune homme’ is not.

 "C'est" vs "Il/Elle est" to say it is/she is/he is in French 

https://www.lawlessfrench.com/grammar/cest-vs-il-est/

 https://www.frenchtoday.com/blog/french-grammar/cest-versus-il-elle-est/

Yes, the 80% rule is: if the verb is être, use c'est.