What can cause a sharp, stabbing pain?

Pinched nerve. Either he has a bruised tailbone (or other injury) or lower lumbar vertebrae is pinching a nerve.

It is actually more stressful to be sitting all day as opposed to standing, so what I am guessing is that after sitting for a period of time, his muscles in back and stomach are working overtime protecting his injured area. They can't keep it up all day, so when they start to give out, he gets that pain. When he stands and walks around, it relaxes those muscles and stretches them a little too - so he can sit again...but they'll just flare up.

He should get checked out by a chiro, or a good masuesse.

It really depends. Has he fallen lately on his tailbone? Has he lifted anything that would cause a pinch?

Nerves are very strange and can send shooting pain into places that are not even the problematic area. I've had similar pains before and it turned out to be a pinched nerve in my lower back.

I'd also look into the fall possibility. A broken tailbone can be unbearable, so it's probably not that, but maybe a fracture could be your(his) source of pain.

Funny I should come across this one.. We just pushed our doctor to get a x-ray done of my wife's tailbone as she was not able to sit properly due to getting sharp pains.

It turns out that she has a slight chip broken off of her tailbone from when she bumped it going down a water slide. She did not feel the pain until a couple of days later when it became aggravated. There is nothing that can be done for it as the chip will eventually break down and dissolve.

Maybe he bumped it and only acts up when aggravated? It may be worth getting checked out.

Dont mean to be graphic here but when you say sharp pain through the tailbone, do you mean like a sharp pain through the butt? I get those and they are muscle spasms and they hurt like HECK!!! I got them off and on for a while and I was really scared and almost went to my doctor. I asked my mom about it and she and my dad told me what they are. It is your colon contracting. Not very pleasing to experience. It eventually went away for me and I have not had one in years. Tell your fiance that if they do not go away, go to the doctor. they might have medication for it or they may want to do a colonoscopy. Not fun to think about but might be needed if they do not go away. Hope this helps.

meant in no way to scare you with my answer. Tell her I know what it feels like!!!

Your fiance might have injured his lower back somehow. He should go to either his primary physician or chiropractor for a consultation and either receive x-rays or mri of his lumbar spine to check for any disc related issues in his spine or any bone or nerve injuries and go from there. Most insurances cover MRI or X-rays. Until them have him ice it periodically for 15-20 min sessions until he can figure out whats wrong. GOOD LUCK

He could have a broken Coxyx or shredded the connecting muscle tissue. Best to go see a doctor.

Broken coccyx yep

he could have a sist on the end or he could have a bruise. it could really be anything. go to the doctors to check it out.

i think this could be a trapped nerve i would advice to go see a specialist

Muscle spasm, maybe?