With the random 'hi' messages, I'd not respond if you're not online. And then if you do happen to be online when he says hi, respond and ask an light question, like "Wow, isn't this weather great?". If he doesn't respond back in a few minutes, say, "Did you want to chat?"
Alternately, if you see a 'hi' message that was sent when you're offline, you could respond back with "Hi. I was sleeping when you sent your message. Did you need something?"
One other thing to consider is that people are spoofing/hacking FB profiles these days. I accepted a friend request a while ago from someone I thought was my husband's cousin. Right away, the person tried to chat, but something was 'off'. The chat started out a lot like your friend....Hello...How are you doing?....etc. Very bland. And no formatting/cap letters, etc. I was happy to chat with the cousin, but there was nothing personal about the first few exchanges and I began to get suspicious. Hubby was there so he told me to ask her a trick question. I did and the person failed it. I immediately defriended that account. So anyway, because of this, if the person can't say anything more substantial than 'Hi', I think you're in the clear with not responding b/c of the danger that you don't know who you're talking to.