As a parent to three children with PKU and six without I have to ask why your husband would be even concerned about gross things like that? I don't want to sound rude, but the reality is when my children have tried to eat their snot I have stopped them – not because of the fear of the amount of protein in it, but because that is a nasty habit. I can remember back to when I was a child many years ago and there was a boy that lived down the street from that (we went to school together) and the kid always picked his nose and ate it and honestly NO ONE liked him. We all through he was gross and stayed away from him. I would never want that child to be mine. Same thing with illnesses and runny noses – I am always hyper vigilent on keeping their nose wiped so they don't eat their snot. To me this is no different then when I have a runny nose – I make sure I wipe it so I don't eat it so why would it be any different for my child? Even my 2 yr old knows how to wipe her own nose because I taught her how to do so. (Now on the flip side phe levels can go up when your child is sick but that doesn't have to do with eating snot).
Eating scabs – again this falls into the that is too gross to allow my child to do category!!
As for sucking blood on a cut – I am sure there isn't that much protein and it isn't going to screw up your son's blood levels from a one time shot of sucking a cut.
Again I am not trying to sound mean, but the things your husband is concerned about really aren't something that he should be freaking out about because the reality is you wouldn't want to encourage your child to do those bad behaviors. I am sure if you brought it up to your clinic they would say the same thing.