If he has impacted poo, the best thing is to brace yourself and help the wee soul out.
Gloves are good (for vegetarian animals , impacted pooh can stink )
you can put a small amount of edible oil round his bum and get a willing volunteer to support him while you ease it out .
Is your piggie overweight? Is he dehydrated? Give him lots of fibre (hay) and juicy veg but not too much fruits or root veg (high in sugar) while he gets his innards regulated. In the warmer weather keep an eye on his water intake.
One of my boars (un-neutered) did huge droppings but he always managed to get them out (I did evacuate him before I cuddled him a few times <boak> ) He was a slender non lardy type so no clue why he got this.
Watch your vet while they Do The Deed then you can see how its done . Good Luck.
He is 5 1/2, don't think he's overweight, how would I know if he's dehydrated? Bottle is refilled freshly each day, he never empties it - maybe drinks 1/4?
He seems to be ok and pooing normally now but I will check his bum each day.
He's been out on grass lots in the afternoons this last week or so since its dried/warmed up, so maybe not eating as much nuggets or hay, but I wouldn't have thought that was the problem as he's never had this before.