I hope you told your son that he is being insensitive and inconsiderate.
HIS Father is recovering from a heart attack! This is no time to have a five year old staying even overnight, and certainly not for a fortnight. (Which is far too long for a five-year old to do without his parents, anyway.)
You are tired after the shock of your husband's heart attack, at least I imagine so, and a visit from your grandson will be tiring.
You don't say you have a district nurse coming in, or a home help, so if you are looking after a convalescent husband all on your own, you certainly cannot look after a five year old as well.
If you are afraid of offending yourthoughtless son and DIl, just say that your husband is taking all yoru time, and that it would neither be fair to him, nor to your grandson to have the boy staying right now.
Has the child no young godparents or aunts and uncles who could look after him? Or maternal grandparents? Or have they all wisely said, "Not on your life"?