Aggravated assault is a second degree felony. Second degree felonies carry a punishment range of 2-20 years prison and up to $10,000 fine. In your son's case, though, we need to be clear that there is no potential allegation of the use of a deadly weapon (the bat) causing serious bodily injury, which could possibly be indicted as a first degree felony. First degree felonies carry a range of punishment of 5-99 years in prison and up to a $10,000 fine. Also, if the State alleges a deadly weapon was used, then it can prohibit the Judge from granting probation and your son would have choose between going to the Judge and hoping the Court will grant deferred adjudication or going to the jury and hoping they would give probation if he is found guilty.
Texas has done away with the distinction between accomplice and principal and replaced it with the law of parties. So the State is not required to prove who was the principal and who was the accomplice. However, the State must still prove either that he is a party to the offense because of his own conduct or by the conduct of another for which he is criminally responsible (or both). In this instance, it looks like the State will not be able to claim that your son was criminally responsible for the other boys conduct, however, if there is anything in his own conduct that contributed to the offense (driving the boys to the location and driving the get away car) then the State may be able to go forward and charge your son with the same offenses committed by the other boys under the law of parties. The real issue will probably be why did your son drive the boys to that location to begin with. Is there an innocent explanation or was it for the purpose of confronting the alleged victim?
Your son needs to retain a local criminal defense attorney sooner rather than later to advise him on how to proceed with defending himself against these very serious charges.