One tick means the message is successfully sent from the sender's device but the recipient has not recieved it yet. (may be because of Internet Off) Double Tick means the recipient has recieved the message in his device but hasn't seen it yet. Double Blue Tick means he has seen the messages. This is a case in the official WhatsApp... Aero WhatsApp Now there are many whatsapp mods that let you read the messages and they will not be marked as blue ticks. Here, we will not know if they have seen your messages or not.
Do you know if he is using the official whatsapp or mod?