Hmmm...yes this is a problem....my rabbits do this but I don't really need to help this(well Spice does and so does Oreo!But Hershey is SO well litter-trained!!!!). Maybe next time you see him trying to pee that way pick him up and flip him around the other way, if he needs to go really bad he won't care what position he's in! Well good-luck!

PS- Bad old Spicey decided to pee on my lap today I gotta get her spayed soon or she'll never be properly trained!