Regarding farm vs city: As someone who has cultured Aspergillus (3 types) since 2005 and just recently am experiencing "twitchy" lungs similiar to asthma which may be due to fungal balls and not the typical allergic response, I've done lots of research. You should be aware that asperg. is in the soil, compost, some ground spices- noting especially ground peppers- in air vents, tap water... A farm of course will exposose your son to environments of bacteria, fungus and mold. But educating people,taking precautions and enjoying the perks of visits to farm life and family can be a plus.
Pseudomonas A. grows everywhere also,on skin, in ears and noses, in showerheads, and beware hot tubs!!It is the common germ that causes ear, bladder infections and swimmers itch. It survives at outrageous high temps compared to lots of "bugs".
I have cultured a weird bacteria Serratia marcesens that grows pink on tile and around taps, also grows in milk.
I have Mycobacterium Avium Complex or MAC also in showerheads etc.
I say sincerely we can do some smart things to avoid exposure, edcate ourselves and others(See the NIMH and other sites.) The rest is unknowns and a bit of luck, good or bad.. My life is drastically different since my CF was diagnosed, preventative care will make a huge difference for your son.
I wish you well in your process of divorce. I have a grown son (unkown CF status since I wasn't diagnosed till I was 44 and I'm 50 now) who grew up with hard issues of divorce. I gently suggest that with time I hope you can get some perspective on your fears and if you can reflect that your son's father and family also love him. Appeal to them to be a team and give your son all the support and love he needs. I've learned that positive expectations on their cooperation can help to shift all the tension and judgements.I hope you have support thru your hard times.