Yes, it is. In this statement, you are not actually addressing any of the lawyer's arguments, you are saying that the lawyer himself is incapable of forming a coherent argument, thus attacking his character rather than his arguments or actions, which is the definition of an ad hominem attack. It is also a very mean thing to say.
(Latin for 'to the man' or 'to the person')
Short for argumentum ad hominem, is a logical fallacy in which an argument is rebutted by attacking the character, motive, or other attribute of the person making the argument, or persons associated with the argument, rather than attacking the substance of the argument itself." (Wikipedia)
Note, however, that this statement could very easily go from ad hominem to not, simply by changing the direction of the statement from the lawyer's ability to form the arguments to the arguments themselves; as you said in a comment below, to say "you have not" rather than "you can not" and give examples of why his arguments are not coherent.