Tony Dokoupil has steered “CBS Evening News” for 16 weeks now. But as the show’s ratings continue to decline, it appears he’s captaining a ship fighting an impossible current. The Dokoupil-led “Evening News” hit its lowest-rated month in total viewers since the anchor took over, averaging less than 3.9 million viewers throughout April. In the advertiser-coveted 25-54 age demographic, the show is averaging 497,000 viewers, its lowest April in the demo this century. But the show is up 2% in total