Related to Instant Messenger conversation.

Hi,I work for a BPO. And we work with american customers. Normally in the morning I wish the customer as "Hi Jack" like that.Now, I continue my conversation for about an hour. What should I type in the IM when I want to contact him after say 2 hours?Should I still say "Hi Jack" or is they any other way? Please note that my customer is an american.