i'm calling bull on his 'explanation'.
email addresses are hacked all the time, but not for this reason, it's usually an automated bot that gets installed on your computer that sends out spam.
they do not contact specific people and write to them in a conversation.
if, as he claims, someone hacked his account just to send out these emails, impersonating him or someone else. i call Bull on this as well, no one would do this unless he was some famous figure or important person and they wanted to ruin his reputation. email addresses are free to create, anyone wanting to meet women would create a new one for free.
being as dumb as he is, he was not only caught by you, lied about it, but he deleted the emails strait away as soon as you pointed it out. you should have printed them out!!
sorry to say but all points to it being him.
check his phone, the phone will also have a 'sent' section for emails (if it's a smart phone) which will also hold any pictures he sent out, replies he made.
check his text/messages in the 'sent' part as well.
Edit_
the likely hood of it happening as he says is very very slim.