I read your question and I understand your concern.
In order to answer your question on chances more info is needed though. It is necessary to know the origin of his
stroke, whether from the heart, atherosclerotic of major blood vessels, intracranial vessels just to name the most common possibilities. According to the cause it should be evaluated whether he is taking the appropriate therapy for his cause.
Of course exercise is good for him. Diet depends on his sugar and
cholesterol levels, if they are normal a balanced diet is enough, consumption of vegetables and fruit.
Whether a second stroke is possible....unfortunately yes even with all these measures some risk remains, he has one major risk factor which can not be changed, age. We can try to lower the percentages of that happening though, through medication and exercise.
I hope to have been of help.