I have read the question minutely. Obviously you are in a dilemma. If you accept compassionate appointment yourself, then you will be able to work for only 15 years, so you will not be able to eligible for pension (minimum eligibility for pension is 20 years).
On the other hand, your son is 15 years of age. So, if he takes compassionate appointment, he will have to do Boy's Service till he attains 18 years of age. Boy's Service is not pensionable. His pensionable service would start only when he attains 18 years of age.
I don't know whether you are getting family pension after the demise of your husband, or not. If you are getting family pension, I would advise you to accept the compassionate appointment for yourself. In that case, even after your own retirement, you will continue to get family pension. If you are not getting family pension, you have no other alternative except accepting compassionate appointment for your son. In that case, you have to guide him in future to continue his studies at least upto Graduation, and then to apply for better job.
Beware! I question everything and everybody.