Hi I know this has come far too late for original author however for others meeting similar Skips my 20 yr experience with this dogs may go on to help others.
Schipperkes are naturally very protective of their home and family so any "intruder" is seen to be a threat the subsequent consequence of this is seen as aggression.
How to deal with this, it is likely that the Skip will not want you to get past the door, be brave under the watchful eye of its owner and their soothing reassurance just push on past go into the room and totally ignore the dog for at least five minutes, by then it will have worked out you are no threat to its family. it may also want to make friends with you speak to him in a low soothing voice, don't hold him around the head remember he has "hunter & ratter" genes in him so holding any dog of this type by the head will be see as a huge threat to it.
Note also you may by now think he's now my best friend however stand up and he will start to display those unfriendly traits again, do the same as when entering ignore him.
continue this routine as often as you go to visit things will get better as time goes by.
To the owner, take your dog out more where he/she gets to meet more people, you need to help him socialise more.