The lab tests, or having no improvement with antibiotics may not prove there was no infection. That is often the case with epididymitis. (I am not saying your husband must have had epididymitis. Please don't get me wrong here..)

Now, with my feeble knowledge, I can only think the knot would be a block, infection or an inflammation from nerve irritation etc. If it is an infection, try the heral tea mentioned in the forum. For the other possibilites try to meet a homeopath. You should pick a experienced homeopath. He should be able to take both at the same time. They are completely natural and anybody should be able to take them ( if he is diabetic be careful because fenugreek can reduce blood sugar). But since you have a good urologist, keep in touch with him and make sure there is no unexpected development there.

Do more test/research as to what that knot might be.