I haven`t seen him. He went to London a year ago. المطلوب هنا استخدام since

It's a year since I last saw him

بس الفكره دى مش فى الصف الثالث الاعدادى

I havenot seen him since he went to london last year

I haven`t seen him. He went to London since 2012

A point is a precise moment in time, for example: 9 o'clock, 1st January, Monday. Since means "from a point in the past until now." Since is normally used with perfect tenses.

I havent seen hime since he went to London
or
I haven't seen him since 2013