Citizen of the Galaxy: Robert Heinlein

In a distant galaxy, the atrocity of slavery was alive and well, and young Thorby was just another orphaned boy sold at auction. But his new owner, Baslim, is not the disabled beggar he appears to be: adopting Thorby as his son, he fights relentlessly as an abolitionist spy. When the authorities close in on Baslim, Thorby must ride with the Free Traders - a league of merchant princes - throughout the many worlds of a hostile galaxy, finding the courage to live by his wits and fight his way from society's lowest rung. But Thorby's destiny will be forever changed when he discovers the truth about his own identity.
A youth who has known only the primitive life of a galaxy slave is purchased by a beggar who turns out to be a man with many extracurricular activities.