Boy people are all over on this one. A lot of this depends on the state they are in, but in most states.
The family cannot drop the charges, nor can they bring up charges. The state brings up the charges it is a misconception that the victim can choose not to prosecute. The court would have the father removed if he tried to defend the kid as an attorney for conflict of interest and perhaps might get rule 11 sanctions against him, although if he testifies during the sentencing hearing he might be able to get the sentence reduced.
When committing certain felonies in most states, if somebody dies during the act it is murder in the first degree and maybe a capital offense. In most cases that would have been manslaughter, but here it is murder. http://en.wikipedia.org/wiki/Felony_murder_rule
So most likely scenario is that he gets charged with aggravated robbery, illegal weapon and first degree murder. He may get charged as an adult, if not he gets charged as a juvenile. Stays in Juvi till he's 18 does 3 years in prison till he's 21 and gets kicked loose. If he gets charged as an adult if he goes in for first degree murder he'll do 30-life. If he cuts a plea bargain for second degree he does 15-30. With good behavior he may be out in 8.
My mom testified against my brothers father (ex step dad) during his sentencing hearing and got his sentence increased from 6 months of parole to 5 years in prison.