I think they forgot each other but then remembered again. Not completely, though...
This is what Yaoiddict said:
"from what I can tell, Ito had already remembered, and he just left Kyosuke in a rehab center to let him try and rebuild his life on his own. But due to the meth overdose, Kyosuke lost his memories. That's why he couldn't recognize Ito when Ito often goes to where Kyosuke works. But Ito still remembers, which is why he's saying he didn't know how to approach Kyosuke."
I'm not sure exactly what happened cuz it's been awhile since I read it, but Kyosuke lost his memories due to an overdose, but Ito-San remembers. However, obviously Itou-san can't just show up and say 'hey I'm your boyfriend from before you lost your memories' so he's just watching from the side..he probably doesn't want to cause trouble for Kyosuke
This is my second time reading this and I still don't understand what happened after 5 years. Did kyosuke actually forget him or wuut I honestly have no idea. Σ(  ̄□ ̄||)