I am very sorry for what your family is going through.
While this doesn't make it any better, the reason he is likely being charged with assault with a deadly weapon is most likely because the state can prove that case easier based on the facts. Attempted murder requires premeditation and/or intent to cause the death- depending on the facts, that could be difficult for the state to prove.
Good luck to you and your family.
Although my intent in answering this question is to aid you in the legal process, my answer does not establish an attorney-client relationship in any way. You should seek the advice and counsel of a qualified attorney in your community to evaluate your legal needs and to advise you. No Attorney-Client Relationship is created without the specific intent of both parties.
I am sorry to hear about your friend and brother.
Murder is a first degree felony that has a range of punishment of 5-99 years or life. Both Aggravated Assault and Attempted Murder are second degree felonies and carry the same range of punishment of 2- 20 years in TDC. Aggravated Assault is easier for the State to prove. Attempted Murder would not change the range of punishment and may be harder for the State to prove.
Attempted murder and assault with a deadly weapon are the same level of offense - second degree felonies. It is easier to prove intent on an assault with a deadly weapon than on an attempted murder. Since the result is the same, it makes sense to charge AADW.
Although I have answered the question to try to help you, you should consult with a lawyer in your area in person on the matter. In addition, my answer does not establish an attorney-client relationship between us.