My husband, son, and I have been walking for a few weeks every summer - this will be our third year on the
Camino Frances, and we hope to finally get to Santiago this time!
My son was 11 the first year, and it was difficult. Every day he got tired and wanted to stop; we carried his pack for maybe 30 minutes a day to give him a little break. (We both got tired and wanted to stop, too, but someone had to be the grown-ups! We were also woefully over-burdened and poorly equipped that first year.) We kept our days to around 20-24km max, with a few lower than that.
Last year, we seem to have walked with a different kid. He rarely complained, was much more confident with his Spanish, left me in his dust, and we walked 25+ days regularly, including one of 32 or so (Burgos to Hontanas). I think we carried his pack once for 15 minutes.
This year, we will be walking with a surly teenager. Oh joy.
Good luck, and enjoy the time with your son!