What don't you understand about this? If he spoke to a lawyer, any criminal lawyer would likely tell him to be silent. And they know best what a person should do in such a situation. He was the last person to see this missing person, the police have seized his car, searched his apartment, put his picture on the Missing poster of Hannah Graham, even as they fully admit that they don't have enough info to arrest him for his involvement in this case, and that all they have is that he's the last person they can trace to Hannah. Lawyer is saying that best thing for him is to lie low--already he didn't do that, racing off the way he did in the car, and now giving LE grounds to book him.
He does have some things to hide. He gave alcohol to an underage female who was looking disoriented on film. He probably did try to pick her up. Maybe he succeeded. He could be innocent of anyting other than that to guilty of killing her and disposing of the body, with a whole spectrum in between, and right now the cops have nothing on him. Anything he gives to them, unless it's proof of innocence is likely going to tighten the noose around his neck. The track record of how black men have been treated in the south in such cases is abysmal, and frightening. So of course he's going to be protecting his hide.
Frankly, in most such cases, someone like him, last one to be with missiing person, car right there, there is something to hide, in fact everything to hide. No surprise he isn't talking unless he has to. The instant that there is a shred of evidence that harm likely came to Hannah, JM is likely to get charged with some serious things, as bad as Murder 1. Of course, he is going to be silent. That is the best thing for HIM to do. Not Hannah, not her parents, not LE, but for HIM. And we should also keep in mind, he might well be innocent of anything more than having that drink with her. I personally doubt it, but it is possible.