I can't recommend it for legal reasons but I have done it. In fact it saved my life and likely lots of money. I used to be broke, like beyond broke nothing and almost lost my life to illness.
I was infected with Borellia Burgdorferi, Babesia microti, mycoplasma, and bartonella henselae from a nasty dirty tick bite (lyme disease with co-infections). I was misdiagnosed for years and improperly treated because of stubborn CDC guidelines and doctors who refused to acknowledge that I may have more than just lyme (borellia). My life was in shambles, I lost my career and was losing my mind along with my health. I took my life in my own hands and took a variety of mexican and veterinarian drugs including doxycycline for fish. I suffered no ill consequences except relief from the systemic infections I was suffering from.
Drstrangelove your child likely had bartonella henselae which I mentioned earlier and I also suffered from. it's commonly referred to as Cat scratch fever. I caught it from a tick which also spreads the disease. The likely reason your son didn't recover is because you were using the incorrect antibiotic. despite months of doxycycline my Bartonella symptoms never went away. it wasn't until I started taking rifampin that bartonella symptoms eased. So the problem was likely with selecting a proper drug rather than the source of the drug that was administered. This is a risk self medicating, you may select the wrong drug which could do harm. If you have the proper drug, I believe whether it is veterinary or human is of little consequence.
I took the doxycycline for almost 8 months along with periodic bouts of rifampin and metronidazole. (doxy and metro were both from fish sources) My liver values stayed stable, and my symptoms slowly subsided from my infections. In my case it was 100% worth it to take fish antibiotics and if I were in the same situation again I would do it in a heartbeat. Some of it is manufactured in the same facility then just pressed into different sized tabs or capsules. Sure there's a risk. But I now have my life back and am extremely successful, I am thankful I had the courage to take my life in my own hands.
I would take American manufactured fish medicine over Mexican human medicine any day. If you're sick with an infection to the point where you're desperate enough to take fish meds isn't the drug purity the last thing you care about? If you're confident it's the proper drug, that should be the main focus IMO.