Everytime he receive fatal blows his reaction looks like he’s in pain but when he fought the needle guy of the six knights of black even having a needle throughout his body he still able to defeat his enemy by hugging it to death disregarding the needles throughout his body.
Does the fountain of youth had the ability to make the wielder immune to pain?
Yes, he feels pain, but most of the time he tends to enjoy it especially when he fights the captain. See season 2 episode 2 which was named "Futari no Ken'ka Matsuri".
Yes, he feels the pain, but it somehow makes him stronger. As normal humans we naturally become a little bit stronger after receiving pain. But due to his immortality he gains much power.
Ban has a form of immortality that makes it so he feels pain, but his "life force" doesn't decrease. Pain makes us feel stronger, like when you're working out. You feeling pain is caused by temporary damage to muscle contractile proteins and/or the muscle sarcolemma. This builds up strength. So Ban's power grows by the more damage he takes.
