This is the million dollar question as far as I’m concerned… and it’s my chief concern right now. I’ve trawled the interwebs on many a Saturday night trying to crack this nut. Never thought I’d ever have to spend so much time reading about male sexuality but I guess this is one of the many joys of PFS 
Here’s what I’ve come up with so far (I suspect none of this will really be news to you guys); There are a few potential angles to explore in terms of finding a solution. Hormonally the consensus among the dudes at sites like AnabolicMinds etc seems to be that penile sensitivity is about tweaking E2 and prolactin (downwards) in most cases and/or increasing test (rarely). Apparently Histamine is also crucial to the “sensitivity” equation and some guys have found success by using L-Histadine (a Histamine precursor). Mechanically, increasing bloodflow is important (PDE5 inhibitors and/or Arganine work well here). A word of caution, Arganine and Histamine run along the same pathway and have a relationship akin to Dopamine and Serotonin so you need to be cognizant of this if you plan on using both simultaneously. From a neurotransmitter standpoint I’ve read that increasing Dopamine (via supps, exercise and/or pharmaceuticals) may be important… as those who have used SSRIs and have symptoms similar to ours can attest to.
Finally, and this is simply speculation on my part, I think some sort of inflammation might be at play here. One of the most bizarre things about this problem (in my own experience) is the appearance of new veins and one in particular at the top of my penis which starts at the base and looks quite angry when erect. More and more I’m feeling like something mechanical is going on here. Or more likely some unfortunate combination of mechanical-prostate-inflammation/hormonal/neuro issues. For me at least this whole disaster began with one painful ejaculation, after which my nads were dead. I’m now up to something like 30% sensitivity. But the reason I feel like this might be rooted in something mechanical is that I often find that my ejaculations feel strained, like something at some place crucial to the sexual experience is literally “pinched” off. It’s a very difficult feeling to describe. I’m also experiencing a bit of persistent discomfort in my nether-regions, along with a pretty constant need to pee So I have to leave open the possibility that the root of my issue may be some kind of CPPS, Chronic Prostatitis etc where inflammation and tightness are at the core of the problem.
I don’t even want to consider nerve entrapment. That’s too depressing… I’ll work my way through that stuff once I wade through the potential remedies above.
Anyways… sorry for the rambling post. I hope it helps someone somehow lol. Bryce I’m with you dude, I’ve read alot on penile sensitivity and it’s kind of silly how little I understand the processes/physiology that accounts for penile sensation. Then again I’ve been more focused on looking for solutions as opposed to poring over physiological resources… might need to reassess how I’m approaching this. More Saturday nights dedicated to male penises? YES! 
Haha… booooooo.