My father had a right side stroke. He was eating and takes medication but now he rejects food. Any advice?

He may be having swallowing issues. My husband after having a massive right side stroke at the age of 48, wasn't able initially able to swallow, so had to have a feeding tube put in his stomach, only to be taken out a week or so later when he was able to finally eat and drink on his own.
Everything initially had to be very soft or pureed though, so you may want to have a swallowing test ordered for your father to see if that's what's going on, as swallowing issues can cause aspiration pneumonia and that is most often fatal.
My husband at the age of 70 developed aspiration pneumonia and almost died. He at that point had developed vascular dementia and the neurologist said that his brain was forgetting to tell his throat to close when he ate or drank, thus causing everything to go into my husbands lungs.
It can be quite serious, so best to get it checked out sooner than later.

Depression is a fairly common side effect of stroke and needs to be treated seriously, his doctor needs to know.

IF your profile is right your dad is pretty young.
Did dad go to rehab after the stroke? If not PT and OT (Physical Therapy and Occupational Therapy) might help.
Is he having problems swallowing? This is something you should discuss with his doctor. There are tests that can be done..
It is possible that his food might have to be minced or even pureed.
If you have a bit more information that might help.
Is he dealing with other aspects of living alright? Or does he need help with dressing, bathing, toileting?
And are you living with him or him with you? Or is he living on his own?

Bring him to his doctor. He might not be able to live on his own. Do not just drop him off but speak to his doctor before the exam.