Van Dyk, a record 10-time winner of the men’s wheelchair title at the world’s oldest marathon, pushed the early pace but lost ground to Hug, the world champion, who stormed clear to win by more than six minutes in 1:29:53.
While the 42-year-old South African was caught by the chasers, he held on to outsprint Masazumi Soejima of Japan for the silver medal in 1:36:27.
April, meanwhile, did well to hang on to the lead group in the first half, with the pace gradually slowing in cold, wet conditions after a fast start.
The pack charged through the 10km mark in 29:43 but reached halfway in 1:04:01.
April began to slip back before the 35km stage, and faded to cross the line in 2:16:25, though he improved his previous position of 15th in his first Boston appearance last year.
Lesisa Desisa of Ethiopia regained the title he won in 2013, securing victory in 2:09:17, more than half a minute ahead of countryman Yemane Tsegay.
Kenyan Caroline Rotich turned up the pace down the lengthy finishing straight to edge Mare Dibaba of Ethiopia by three seconds, winning the women’s race in 2:24:55.
Tatyana McFadden of the United States won the women’s wheelchair event in 1:52:54.