How do I relax my father when he wakes up at night from anxiety disorder and unwanted OCD thought making him very ill and causing sleepless nights?

Meds and CBT. Nocturnal ocd like nocturnal panic attacks can be especially tricky because you don't have any way to see it coming. Make we have it correctly identified and that he get educated on various treatment techniques. Meds and cbt can be helpful.
See a Professional. In the long run, it's important for him to get help & avoid these symptoms altogether. Meanwhile, pay attention to sleep hygiene --- avoid caffeine & nicotine especially late in the day. When he does wake up at night, see if he can distract himself perhaps by listening to some music he likes or reading.
Sleepless night. The first question is is he being treated for this condition? If he is being treated for this condition why is it not working? If he is taking medication has genetic testing done determine if the medications work? Is he taking other medication in the condition worse or even drinking small amount of alcohol? He is prescribed medications really taking them? I second opinion maybe very helpful.