this is just my understanding. haru wanting to call his ex is simply for a closure. the same thing the ex wanted as kiyo said she wanted to properly apologize since she just left him. it seemed that after the shelter found out their relationship, haru got kicked out and she left (likely fired) so they never had a closure.
haru himself thought that he never thought of her until kiyo brought her up. meaning he had moved on. it's not too rekindle the relationship.
as for ch 17, yes, i think he did slept with someone. we've seen how moody he got when it's close to the memorial day. it seems the survivor guilt and feeling of loneliness is at its peak. it's been heavily implied but haru's past relationships seemed to be more of a bad coping mechanism for a sense of belonging. to cope with his loneliness than actually liking them. add to that, ren wasn't there with him. he was away on a summer camp. so he's alone in the house at canada during his parents memorial day. it was probably too lonely for him, and he lapsed to his usual bad coping mechanism which is sleeping with someone.
as for if he did like ren—romantically, i assume—during this time. it's kinda complicated, no? one of the twins mentioned in later ch that haru already fell for ren when he chased after him to canada, but at the time, haru himself never really thought how he actually felt for ren. all he knows is that, "i love ren, my little brother."
the story progresses to how when ren declared he would simply be his brother and haru wondered how is this any different than their usual relationship. iku asking what exactly is their relationship(it was smth along this line) and haru answering he never really thought of it. (but during that question, he did already acknowledge his romantic feeling for ren.)
so, going back to the question if haru already liked ren back then? i think yes, he did. but he had yet to actually realize or process the nature of his feeling. he just knows ren is precious to him.
When I read chapter 6, Haru's friend persuaded him to call his old girlfriend, at first he seemed to hate calling her because of the past, but on page 36 I saw Haru almost called her. So I don't understand why he did it. He wanted to reunion with her and loved her again? (I have watched that episode of anime many times so I feel confused because I think he loves Ren and don't understand why did that)
Chapter 17, in Ren' flashback, Haru went out and came back the next morning with someone's scent on him. In the anime I watched, that was around the time of his parents' memorial. He often went out drinking to cope with his sadness. Did he sleep with someone? (Like I said earlier, I feel confused because I think he has loved Ren already and don't understand why he did that. So in that time, did he love Ren yet? Or he hadn't known his feelings for Ren?)
Those things were the reasons made me feel confused many years and I still don't like this manga. Can anyone explain them for me?
I am not British, so maybe my words are wrong, please understanding for me. Thank you