[Ria]

As he is 6 weeks, and your vet has tested for.mites and all other information you have given I think its might just be quilling, they loose LOADS of baby quills in this period and it is perfectly normal.

Although I have a couple questions just to be sure.
1. If you look closely at his back between can you see new quills growing (they look smaller then the others as they are just coming in)
2. The oil do you rinse it off or not
3. How much you bath him
4. The scratching would you say is excessive scratching, odd scratching or in the middle

 

[Ria]

12 weeks is still the time they can be quilling.

Yea thats fine when ever you can check just let us know.

I've always been advised to rinse off oil that goes on their backs otherwise it can cause irritation, but I'm not sure if its the same with this coconut oil. I know when put on ears for helping ears its not washed off.
How many times have you put the oil on his back?

Try not to fully bath more than once a month less if possible. Use damp cloths wipe stuff off quills when needed.

Definitely sounds excessive, I cant remember what else other than dry skin, irritated skin and mites cause this sort of scratching. Quilling can cause a small amount of scratching depending on the hog but this is too much for just quilling.