Solaire is canonically NOT Gwyn's son, Miyazaki has directly responded to that. The point of Solaire's character is a completely normal man powered by his faith alone (see his items). Solaire was created responding to the DeS criticism that all religious characters were evil. Cut content for DS1 strongly implies that Andre of Astora (blacksmith) was originally going to reveal his heritage as Gwyn's son in Anor Londo, meeting the player there and opening a shortcut.