Looks superficial to me,the lameness is probably due to it being sore
I agree that he shouldn't be stalled,moving around is best to keep swelling down and help it to heal quicker.
I would just treat it with a good antiseptic scrub like hibiscrub ,then I usually apply some savlon...I'm a treat it at home kind of gal..
As long as it's not hot and the lameness goes away he should be fine.
In my eyes there is NO excuse for not cleaning water buckets it's just sheer laziness,sure we can clean them ourselves but when you think it's being done by the BO we don't know till its manky lol.
Glad your moving to a better place soon.