Dragon Stand, world full
McDingus – DDLG guild – Stormbluff Isle
Does your son have a different friends list? Belong to different Guilds? Belong to different Home Worlds? The game may be putting the two of you into different instances of the map, depending on the algorithm’s requirements. Certain instances will have places saved to allow joining friends, guild-mates, etc.
It absolutely should let you join your son in a party there, though.