Sorry to hear about your dad :(
I too had a right side stroke, left side paralysed, 16 months ago.
Generally speaking arms are slower to come back than legs. When you think about it, the arm does much more complex work than the leg. It is possible, depending on the location & severity of the stroke and also how much effort he puts into it. To get the arm moving again requires the developmernt of lots of new pathways - is lots of tiring work. I have always found it far more tiring working on my arm than my leg - brain dead sort of tiring, just wanting to close my eyes & shut eveything out.
You don't say when he had his stroke, please remember that stroke recovery is a long slow process - so a couple of months (if that's the case) is really early on. Please tell him to keep up the hard work.