Can I address someone in an email with their first name if they sign with their first name?

I'm planning to do an internship and I usually address my supervisor as Mr. X, however, he signs his emails with his first name is that means I can address him with his first name ?

Note: I haven't met him yet.(if that's makes a difference)

This is more an etiquette question than an English question, but the rule is easy enough.

When in doubt, be consistent. If you would address your supervisor as "Mr. X" in person, you should do so in an email as well. Conversely, if you would address them as "Y" in person, address them as "Y" directly as well.

It is possible that your subject may have their own particular preferences, and once those have been expressed it is good form to follow them, but until you are sure as to their preference you should err on the side of consistency. (It would be entirely appropriate to ask them how they want to be addressed.)

How someone signs off an email indicates how they want to be addressed. Hence if someone signs off with their first name only, this is a clear invitation for you to address them using their first name.

Continuing to address that person by their full name nonetheless is a sign of insecurity or possibly rudeness, and should be avoided.