I believe its different for everybody. Maybe the brother smells scents different than shingo does. Thats how i want to see it.
Honestly, if you're looking at it from an actual animal perspective, the brother would be right. Creatures, such as cats, only really excrete pheromones when they're looking for a mate. I think that the brother was probably right. Keiichi just seemed to stop caring about Shingo.
Keiichi was spending all his time with Angelo, knowing that Angelo wanted him for sex. He knew that Angelo's scent was getting all over him, and his first thought when he came home was to shower, which is something that only guilty people tend to do. Maybe he didn't physically cheat, but you could definitely look at it as an emotional affair.
His relationship with Shingo was 90% sex but as soon as he was spending so much time with Angelo, his sex life with Shingo all-but-disappeared. He didn't care that he was hurting Shingo. It seemed like their last time before he went to America was more about him trying to make Shingo believe that he still loved him.
I am a little confused with the whole "scent" thing, like keiichi and his brother explained it in different ways actually opposite ways. So how do you know it is not keiichi who lost interest and why was he hanging out with Angelo even tho he seemed to hate him?