I don't think slender is a particularly common word to use for men. You could use it though. Slender doesn't just imply thin though, it also tends to imply delicate or perhaps graceful as well. It's more commonly used to describe features rather than general state. - A slender neck. - Her slender waist. - Long slender fingers. The overall body shape of men is usually described as skinny, slim or thin.

yes, you can use slender to describe a man. for example: “have you seen john lately? he’s very slender now.” OR “mike is a slender boy so that coat may not fit him well.” “slender” is just another way of saying thin. you could also use “slim.” slender, skinny, slim, thin can all be used to describe people, animals or objects. does that help?

Talking about body shape and type could be a double-edged sword if you don’t know the precise meaning and connotation of certain words (in this case adjectives). We as language learners can sometimes make mistakes a tip for this situation: use synonyms if you aren’t sure what you’re saying :))) In this case, Slender is okay for describing slim people in an attractive way; it’s frequently used as far as I know.

For a man it is more common to say: Slim or fit

It's not uncommon. It can be used to refer to anybody. It means "thin" but there is a positive connotation to it, like thin in an attractive or aesthetically pleasing way.