Rabobank rider Bos edged John Degenkolb (Argos) and Heinrich Haussler (Garmin) on the line after the 188km stage from Riemst to Genk.
Orica-GreenEdge's Jens Keukeleire retained the lead in the general classification.
At the Tour de l'Ain in France, Yauheni Hutarovich (FDJ) won the second stage, from Lagnieu to Ambérieu-en-Bugey, ahead of Andre Schulze (NetApp) and Sebastiaen Chavanel (Europcar).
Hutarovich leads the general classification by 10 seconds.
- Yauheni Hutarovich