How, in a morally acceptable way, does one ascertain male infertility or its cause?
In this case the patient has a blockage. If the blockage is removed the tubes will be damaged rendering the patient sterile.
It seems you are assuming that, just because the operation to remove the blockage will cause sterility, the operation is immoral. This isn't necessarily the case. Is the blockage causing the man to suffer? Given the area where the blockage is, I imagine that the pain must be pretty bad.
If you are looking for a different procedure to both remove the blockage and preserve him from sterility so that he may procreate, that's very laudable. But if you are worried about whether it is immoral to operate for therapeutic reasons where sterility will result, I would speak with a priest to alleviate that worry. The principle of double effect would seem to apply here.