A quick Google throws up
THIS article about the Rouen, and quotes;
"Both 700c models are available to purchase in 43cm or 48cm frame sizes. The 43cm frame is sized for riders of height 145cm - 150cm (4' 8" - 4' 11") and aged approximately 11 years and over. The 48cm frame 700c is designed for taller riders, 165 - 170cm (5' 5" - 5' 7"), and will be a last junior bike before progressing onto adult road bikes."
155cm makes him too tall for the 43 but too small for the 48, (according to the above anyway). Best to get him to
Halfords to sit on it and see what's what IMO.
Hope that helps