There is simply no PTR record associated with that IP address, this is configured optionally by an ISP so may or may not exist.
In no way does this indicate if a person is "online", that's just silly.
You can use
whois(1) to discover some information about this users ISP, and perhaps some contact addresses if you wish to report abuse or something, but he may be using a proxy or tunnelling.
http://en.wikipedia.org/wiki/Reverse_DNS_lookup