Being there for him is the best thing you can do. I didn't want conversation, it was too much for my damaged brain to tolerate, but it was great that my wife would sit with me, read, go for coffee and for lunch. She was of practical use for reaching my bedside cabinets etc and for checking where I was with progress etc. and clean PJs.
Perhaps you could help Dad to carry out any exercises that have been prescribed by the physio. Generally, at three weeks, we just want to sleep. Stroke recovery is another world. The time scale is enormous and that's hard on carers and people who care. You should pace yourself.
One little thing, perhaps you could write a stroke diary, indicating what Dad can do at each visit. I have kept a stroke diary and find it very useful.
I am so sorry for you, and for Dad.Its a cruel illness.