I'm kind of concerned???

Ok so I know this isn't about any of the topics, but ok so I turned 18 a bit less than 6 months ago, and an old family friend just started messaging me inappropriate messages like "I'm naughty" or "I know this might be creepy but... I like u" and then he gave me his phone number, and he keeps asking me "ur 18 right????" And I'm really uncomfortable and mind you he his in his 50's AND HE HAS TWO DAUGHTERS A YEAR YOUNGER THAN ME. Gosh I just hope he isn't that sick and says this shit to them.. but I'm afraid to tell anyone bc they'd say "you're an adult now, you handle it" or they wouldn't believe me because he seems like such an innocent guy, and sweet too, until this. And I guess I don't even know what to say to him or what to do about it, because I don't want to cause tension in the family, or for all ik he would get worse idk, and I haven't responded to his messages at all, they are all left unread but it's still concerning. Any advice...?