I really wouldn’t recommend him going from a 4km run, to an 8km with a hard last km as a start point to improve his 2kmLong runs help. Build on your base level of aerobic capacity and you'll fly. That's best done through tempo runs and threshold runs to really start getting comfortable with being uncomfy.
If you have time - stretch the Friday run about a little longer. Doesn't need to be super fast, maybe at about 65% effort for 7 or 8km and try make your last kilometer quick.
You could also make your Mon/Wed runs on hills or grass which is a sure fire way improve quick.