Poor boy. It is one of the downsides to boars unfortunately.
My DD and I had brothers, one was 4 when he died , no trouble, magnificent losenge shaped glossy dropping (I sound a bit Dr McKeith 
)
Our other boar had a dodgy bottom, not sure if it was impaction, he managed to get his droppings out on his own without too much trouble.
They were without a word of a lie, the size of a Fererro Rocher 
. I took one to show the vet (he was impressed I'm sure, never seen one like it)
A couple of times we had to help him, you need gloves, some wipes or similar and sort of ease it (not squeezing too much) You can put some edible oil like almond or vegetable in the anus to help it (but that gets a bit messy)
It stinks BTW. Once it smelled worse than dog pooh (hoe a vegetarian guinea can make this smell I don't know)
In between his mega poohs he did normal smallish losegne poohs. (He was the loveliest piggie from the bum forwards)
Our GP3 now deceased and present boar GP6 are neutered so we should bypass this.
Keep them as active as possible and keep their weight controlled. Lots of fibre and water on tap.
Hopefully having his backside sorted will clear the backlog and he'll get back to normal but keep a close check, especially in Summer, you don't want flies round his poor bum .