criminal law

Hi,
I am a mother of a convict in one of Spanish prisons. My son was indicted of drug trafficing to 9 years and has served 1/3 of his sentense. Thanks to human penitenciary system of your country he's been on his first furlour last months for his excellent demenor. He shows his desire to change his life to better and all the stuf of the prison thinks high of him. The lawyer in prison told him that he can be subject to expulsion next summer and return to the USA (he is a US citizen) without serving the rest of his term in Spain.

Will you make clear the pros and cons of this process?
What consequences can it have for my son in the US?
Is it better for him to serve the whole term in Spain?
If he chooses to stay in Spain and study at the University, what can we do to settle him in your country legally?