Have you fed him anything like Beetroot? Is it coming out orange or changing to a orangey/pinky colour? Are they crying when weeing?
It is worth taking a Urine sample to see if it's coming out milky then changing or coming out red... Sometimes as wee drys it can turn a orangey/pinky colour due to the iron oxidizing... sometimes this can happen after eating food with a lot of iron in it.
To get a urine sample you will need:
1 x sterile pot
2 x 1ml syringe
1 x large Tupperware or bowl
The way we do it is to carefully syringe piggle some water using one syringe or if your not comfortable with syringing water - feed them some very very wet veg, cucumber, lettuce, soaking wet celery Leaves are good for this.
Then place your pig in bowl or large Tupperware and wait for nature to take it's course...
After piggy has had a wee, suck the wee up using the other syringe into the sterile pot.
It will be worth taking a urine sample down to the vets with you so the vet can test it. Place pot in fridge until you take it to the vets.