Use every opportunity you get flirting with him and be sure to ask him questions about his past. Sometimes the additional opportunities emerging flirting to do. Take your time with him. In Act II, you will find the book in extraneous Shartan at nightfall. It came closest in a bag. Be sure to pick it up, as her two gifts need a little power in the novel. When you give, be sure to tell him that you will help them learn to read.
Although I have not tried all the results of things you can say, personally, I tell him he must begin a new life, rather than pick up where he left off. It is difficult for someone with amnesia to pick up where they left off because they can not remember their past, and even when they do, it will be more alien to them than what 'meaning they lived. Thus, the collection of those options made sense to me.
When you're 50% friendship with him, takes a look outside the region Kirkwall. You have a slave Tevinter. If you do not progress the plot a bit and come back. You'll learn his master's apprentice is there to hunt. Go after it. When you've found the boy alone, give some money to start his own. Do not forget to take his armor upgrade at this! It will be the only time you can! When the woman down, so he can choose himself what he wants. Then comfort. He will be disappointed, but it does not matter. It will find you and apologize.
However, things will happen to Fenrir, he can not take for it is with you and he will "end" romance. I usually tell him that I help him, we can overcome this obstacle. But he will leave the relationship, whatever. If you played your cards until this point, Fenrir wear a red ribbon around a wrist. :)
Act Three come around and visit Fenris is his desire to meet with you to her sister. Of course, if it is a trap, and Fenris must take down his former master. Then, we give ourselves to him, or tell him not to kill his sister. In any case, it's over. I've done it both ways, and do not seem to affect your relationship. He says he is alone, tell him that he is to you. It seems that the fight, then tell you who needs to get out of there.
Appointment to meet again at his home circuit. Encourage them to do his usual flirting and things will be much. Talk to him again later, and you should find Verric Aveline and say you need to get a place. Talk to him and tell him he can stay with you. Not appear in your house, like other comrades still asked to move, do not worry and I think poor state of their romance.