Dlee see my thread urine scold in rabbits http://www.sheffieldforum.co.uk/show...ht=urine+scold
Fiver had this and its caused bu a Urinary Tract Infection (or UTI his was cystitis). You need to get him to the vets so he can be treated with antibiotics and painkillers.
What is his diet like? What foods does he eat on a regular basis? If a rabbit eats too much calcium his body cannot excrete it effectivley and it starts to form stones in the bladder or a sludgey layer. This is more serious and can be diagnosed by X-ray and then flushed out (sludge and sometimes small stones) or surgically removed (stones).
Take him inside and wash the affected fur, dry it well and then apply some vaseline to form a protective barrier as it will start to burn his skin. It may also be worth trimming the fur down around his groin so it cant hold as much urine.
Do not put him outside until he is completely dry!