While there is limited mobility due to an injury, you will need to change the area is sitting more than once a day. If he isn’t moving around as much, then that can explain why he is appearing to be getting wet areas. Clean him with a cloth as you don’t want urine scald.
Keep an eye on his pain levels - you’ll have to see how he is, if he is in discomfort then once a day pain meds possibly won’t be enough - piggies metabolise pain meds in about 12 hours and knowledgeable vets know this and prescribe pain meds twice a day. Keep an eye on his weight to ensure he is eating enough - being in pain or discomfort can stop them from eating enough hay (so you will see a weight loss during his weight checks).