Blowjobs Make My Mouth Go Numb???

Let's begin by saying that my boyfriend is NOT the most sensitive guy out there when it comes to his penis, so blowjobs are a total pain in the neck (literally). Plus, it doesn't help that he's quite well endowed in both the length and girth areas. 
From all the girls he's ever received a blowjob from, I'm the only girl who has ever been able to get him to orgasm from it, and even then I've only been able to do so a handful of times.
I'll go for so long that everything hurts--my jaw, my lips, my neck, my arms, etc. And I'll go for so long that when I'm done (whether he's came or not), my lips have gone completely numb from the pain.
But the weird thing is, when I DO ger him to climax with my mouth, his sperm makes my mouth go numb, and I won't be able to feel my tongue or lips for anywhere between 3 to 5 or even 10 minutes.
Does this happen to anyone else? Does sperm make anyone else's mouth go numb??
Also, does anyone have any good tips for being able to make him climax by way of oral? (Without the assistance of vagina, haha)
I've tried everything! Slow, fast, with hand, without hand, massaging his testicals (he doesn't really like them touched), rubbing his gooch (it does nothing for him), fingering his bum (it doesn't do anything for him), deep throating, focusing just the head, sucking hard, sucking soft, rotating, licking, teasing his urethral opening, etc. etc. etc. Nothing works!!!