Yes, "I have known him for 10 years" is correct. If the process is continuing from the past to the present, using this form is correct. If it is something limited to the present (or a question that does not inquire of the history) you would use only the present tense. So if you were asked, "how long have you known him?" The question is asking of the history and presuming you still know him. Thus you would answer, "I have known him for ten years." If you were asked, "do you know him?" The question is only asking about the present so even if you have known him for ten years, your history with him is not necessary to answer the question, therefore you would respond, "Yes, I know him."

We use present perfect to connect the past to the present. My students in Slovakia usually ask "how long are you here?" when they want to know the time between my arrival and now. The problem is, "how long are you here?" means the total time I plan to spend in Slovakia. The answer is the same, even if they ask the question three months from now. "How long have you been here?" means a starting point in the past, up to now. Present perfect is something you have to use if you want to progress past elementary English.

The English present is used for events happening now, or for repeated events. I read the news every day. I sing only in the shower. If I say that I know him. I use the present tense without specifying a duration. I knew him ten years before he was elected mayor. Past if I have a related event. I have known him for ten years. I met him ten years ago and we are still acquainted over a period of time.