I read that sperm cannot repair DNA damage. So does this mean that if your sperm has DNA damage, then all of your sperm (whether present or future) has DNA damage? Or does it just mean that the present sperm has DNA damage, but new sperm will be healthy?
Male gametes (aka sperm) is produced by meiosis of numerous germ cells (gametes precursors). It is unlikely that all germ cells have damaged DNA so the newly produced gametes should be healthy. This also implies that if the DNA inside the gametes was damaged after they were produced, then again, the new gametes will be healthy.
As a disclaimer, I am talking exclusively about damages to the DNA, it could very well be that gametes are unviable for other reasons than just spurious DNA (e.g. low mobility). In this case, viable gametes are unlikely to be produced unless the reason underlying the original production of unviable gametes is fixed (e.g. via medication).