infact this is quite a common problem for lop rabbits
ive had 2 rabbits both lop and one had this problem
it eventualy turned out he had stones in his bladder and had to be put to sleep
the right diet can solve this problem tho
and athough the myth that rabbits love veggies and should be given all the time is actualy bad
if you feed him veggies stick to harder dryer veggies and only on the odd occasion like 2 times a week or sommit otherwise they will only eat the veggies and not there food
i will recommend burgess exel pellets and burgess exel forage
give him plenty ofthe forage grassy foods and his stinky bottom problem should settle
you must check reguarly that his bottom area is spotless or as someone mentions above he could be attacked by flies and stuff even more so if its an out door rabbit
but mainly this problem happens because rabits tend to eat only dry grassy foods in the wild and dont need veggies only as treats and supplyed in small portions
lettuce and cucumber ect are a no no but things such as cabbage and carrots shouldbe given on the odd day in small portions or the rabbit will polish it off and it bacicaly shoots out the other end as a big mess bacicaly
projectile pooing as my vet reffered to this as lol
anyways i think less fresh veggies and more fibery grassy things will put an end to it
also you might need to get your vet to give the rabits behind a little trim on the clippers
this will help avoid it sticking to his fur
also get him checked for mites
as this can happen when they have mites
you can check your self by parting his fur andlook for flaky skin
and white flaky stuff and exessive hair loss or bald paches
this will indicate a mite problem and your vet will have a treatment for that
hope that helps
pugz