He sits on the toilet and refuses to pee. What can I try?

It could be part of an enlarged prostate with growth inside the tube or just simply a preconditioned muscle waiting for him to stand.

Running water gently in the sink often helps someone to let go of the voluntary sphincter that releases urine. If you were never "that person" who couldn't pee when someone else was in the next stall, then you don't know what it is like when anxiety simply will not let that sphincter open. The less anxiety there is around the whole thing, the better. Breathing exercises also help. A big long deep breath in, and then letting it ALL out slowly, and at the end that urinary tract sphincter will just open up. Wishing you good luck. When I worked in OB as a nurse the patients were often so sore after birth, and so "up tight" they could not pee. Leave them alone and they would topple off the toilet. But get them a bit side tracked with running water and relaxing the breathing often helped a lot.

I should ask a gentleman about this. Presumably your husband has spent most of his life peeing standing up. It's peeing sitting down, the much more recent routine, that isn't making sense to him, could it be, do you think?

Have you ruled out a medical problem? My dad had trouble with starting flow and bladder spasms.