fixing doesn't NEED to happen with boys the way it does with girls (females live an average of half as long if they aren't spayed due to their proclivity for cancers of the reproductive system), but it still helps with litter box habits and puts a stop to obnoxious behaviors like spraying urine (which yours may or may not start to do at some point).
have you checked our library of rabbit-savvy vets to see if you can find one in your area? try these two threads to make sure you haven't overlooked a rabbit-savvy vet in your area - would be good to know if there is one in case he gets sick at some point in his life!
http://rabbitsonline.net/view_topic.php?id=47393&forum_id=9
http://rabbitsonline.net/view_topic.php?id=36134&forum_id=9
5 months old is a "teenager" - not a baby any more, but not quite an adult yet. it's entirely possible that his litter box habits would improve slightly with age even if you aren't able to get him fixed.