You can use Swarfega (the greasy stuff in a tin) rub it in then use neat shampoo to break it all down. You'd need to rinse well maybe shampoo again.
I've never used it myself , I'd worry in case they managed to ingest it , but if you were doing the whole episode at once , you could make sure he doesn;t .
Any oil will break down oil - so you could use olive or vegetable oil, (I used to put coconut oil on GP6 dry bits) . Massage it in well , break down the manky bits with your fingers
Then neat shampoo , massage , rinse.
We've only had one greasy boar , that was our GP1 , big golden agouti smooth boy (uncastrated).
Our Teddy and Rex never had (but they were castrated)
Do you find when you wash him that he's dry in seconds
Its like the coarse coats are Teflon !
Is he very "boary" ? Our GP1 was , the others , not so much .