Hello Jasmine, I'm sorry to hear that your father is ill. You should consider seeking advice from a medical professional if you are both worried about the potential for Alzheimer's.
What I will say is that, yes, I have experienced something like what you describe. For me, lack of concentration and general sluggishness in thinking was most pronounced when experiencing prolonged bouts of pain. In general, the longer a flare lasted, the worse the "foggy" effect would become. I suspect that the concurrent low mood, malnutrition, lack of sleep and fatigue contributed to this. Perhaps symptoms like these may be contributing to the problem you describe.
I personally did not perceive any such effects from taking Humira. I found that the effect lessened as the Humira worked to reduce the inflammation and symptoms improved.
Personally, I found this to be a particularly distressing effect of the disease. I greatly appreciated assurances that this symptom would almost certainly lessen as inflammation was brought under control. Providing reassurance in this regard may be helpful.
For a more objective account, here's a press release detailing research into this. It's certainly not just him reporting cognitive symptoms! You can find the actual paper in the references, if you'd like to read it -
https://www.ueg.eu/press/releases/u...s-impact-of-crohns-disease-on-brain-function/
I hope he begins to feel better soon. Best wishes.