My understanding is that a half-dragon can't use the spell disguise self to appear human. A half-dragon just has too many limbs for that to work properly, not to mention technically the dragon is no longer humanoid in type.
Type: Creature type changes to dragon. Do not recalculate hit dice, base attack bonus, or saving throws.
The disguise self spell says, "You cannot change your creature type (although you can appear as another subtype)."
Can the disguise self spell allow a half-dragon to appear as another Medium dragon, a wyvaran, a drake, or anything similar to dragons?