No words can explain im so happy thank you sensei for this final chap bet them...

i didn't understand what happened to him?

Kyosuke remember his feeling of love to Itou san. Not all his past. Only itou san

yeah hes touch and feelings and the emotion itself :3

i mean why did he lose his memory?

Did you read the manga? When kyosuke & itou san were captured, itou san mind about kyosuke has been erased while kyosuke was under machine to erase his memory as well. Kyosuke begged to not kill itou san and let him keep his memory about itou san, so the boss decided to kill kyosuke with unfinished memory erasing but he was given too much drugs and he lose his memory about his past. It only said he lost his memory due to overdose (the drugs that given to him by the assassin boss), but it also high possibility because the memory eraser machine has it effects worked

i was actually thinking that the drugs that were given to kyousuke is part of the memory-erasing procedure. and yes, as you have said, maybe the memory-erasing was half-finished because it's first part is administering drugs and the only thing left is the electric waves, smth to totally erase his memory.
if it's like that, then it makes more sense.

Yeah well explained :3

yep this too explained it well :3