Awww welcome to the downside of boars .
They do tend to get a bit saggy arsed in older age , and 6yo is approaching Old Duffer Territory .
Unfortunately , the only thing you can do is give him a Helping Hand . It bloody stinks sometimes , not like chopped up digested vegatables at all 
Our first two boars were uncastrated ( impaction is less likely in castrated boars) the little boy ( Abbysinian ) did poohs the size of a Ferrero Rocher at times ! No trouble passing them, if I lifted him up and there was one there , I put him back till he;d done his bit , usually just walking along .
Couple of times I did the bum dip in warm water , give him a squeeze then dry . You can put a tiny bit of edible oil round his bum if you worry about sticking ( vegetable or sunflower oil)
He did do normal poohs usually . I did show the Guinea=Pig Vet and he said it was "unusual" (polite
)
Make sure he has enough fibre and nice hay . Of ours had a day grazing they had hay/pellets/water for supper (much to the other hogs disgust) /
Ours had unlimited bedding hay (except in the height of summer they had hay in looroll tubes to eay ) and loved Readi-Grass and the Jolleys PetShop hay for eating .
Try and keep him active and watch his weight (lazy little eating machines that they are )
Our two later boars were castrated as they had PigWives so we didn't have any problems with them
Is there a photo of the little boy ?