Hi,
Thanks to care of your adults children’s souls.
Yet, I want to underline some points:
1: You cannot known for sure if there is some infertility issues on your son’s marriage. Sterile couples can be seems healthy and not overweight. You cannot detected them by just seen them. As a conclusion, this possibility cannot be exclude.
2: If there is infertility, you cannot jump on the “logical” conclusion that they should have some exams now. It is good that you want them to see only doctors who respect our catholic morality, but you don’t have push them to do it. They may perfer to wait, before trying something. They may prefer to not know at all, or don’t want to do something. Even if we try to “save” some morality aspects, exams are an invasive process, and every people have the right to find them immorals, or don’t want to do them. It is not a mandate.
3 : if they use contracpetion, I doubt that your son don’t know it! It is even more possible that it is of common agreement. Your son is not innocent until the proven contrary. It may him who want it and not his wife, who know?
4 : They can have issues on their marriage that they don’t speak with you, who know?
5 : They acn wish to delay pregnancy, with contraception or not, who know? They may not care of the catholic doctrine?
There is many possibilities!
My advise would be : you can ask your son, if you want. You take a risk, it is your choice. But not your daughter in law, unless you have a strong relationship and she is very open to discuss thoses topics. If they have really a problem of fertility, you should be very prudent, pray for them, ask them if how you can help, but not push them to any decision.