The science on neutering is unclear imo and what seems to be happening is, as it filters down through into popular understanding, the nuances are being lost in favour of an overly simple 'neutering makes fearful dogs worse' message. I get why that would happen, but it having studied this for the last few years, I don't think there are enough (any) clear studies showing a causation link. Which is not to say it does not exist, just that it is a complex thing to prove and there is not yet enough research. Plus, every dog comes with it's own experiences and genetics - making it tricky to really know what the cause of any behaviour is for sure.
Testerone has been linked clearly to confidence levels in multiple mammals - suggesting a dog with less testosterone might be more fearful. However, it is also linked to the threshold for aggression, suggestion a netutered dog might be less likely to use aggression at a lower level of arousal than an intact one.
There have also been studies seeking to seperate out the different causes of aggression and suggesting neutering may reduce same-sex aggression but increase stranger and female-dog directed aggression. Again, those studies found a correlation, which is not necessarily a causation - but might be.
In the end, I suspect the answer is for each owner to work with their vet, their trainers and their own research which includes seeing the dog in front of them - and make the best decision they can for their dog.
Medical pros and cons are a bit clearer but do not come down in favour of one way or another. e.g. Bone cancers occur more in neutered dogs than unneutered. So it's also not so simple to say there are medical benefits without also saying there are medical consequences.
Either way, this is an 18 month old dog. His testosterone is naturally at its highest point right now. As he ages, it will decline naturally anyway and this might be enough to stop the the crusty spunk problem - eventually 
FWIW I think some dogs do have this happen and it's normal - if very icky. In fact, I once had someone complain to me that their dog left it splattered up the stair walls as his willy kinda bounced from side to side when he ran and up and down the steps - thus flinging it onto the wall.
Perhaps washable mats where he likes to lie?