How do you know if a guy is infertile and what makes him fertile?!
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He can take a test, but would need his doctor to refer him before he could do it.
This would involve him *ahem* producing a sample (i.e. masturbating and collecting what is produced), and taking it to the hospital where people in a lab would look at the sperm under the microscope to see how many sperm there are, and how much they are moving etc. You have to take the sample to the hospital within an hour of producing it, and you have to keep it at body heat, so most people opt to produce it on-site.
As to what could make an infertile man fertile again, I think if he was producing no sperm at all, there's nothing that can be done to make him produce it again, but if there is some sperm, there are some (medical-type) procedures to help get the sperm to the egg.
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You can't really tell without going to a doctor/fertility clinic. There, they can test the sperm and see how motile they are and how many and so on to determine how fetile he is.
Grass won't grow on his back. Fertilizer may help.
he has to see a doctor and if it runs in the family he could be or if he ever smoked pot, that increases his risk
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