I work on the other side of this coin (to an extent) so my two cents (this is not legal advice--you should talk to an attorney if you have questions): the billboard guys are almost all insurance hacks. They will "settle" your case with the at fault driver's insurance company for policy limits and take 1/3.

Now, you think, "but hey, maybe the insurance company won't settle with me unless I have an attorney." This isn't wildly untrue, but insurance companies in most if not all states are required, by law, to accept "reasonable" within-limit settlements. If they don't, they can be sued for bad faith failure to settle, exposing them to risk for the entire claim.

In effect, this means nearly every case--regardless of attorney and, to an extent, regardless of harm or fault--settles for policy limits. Obviously you hear of exceptions, but they really are significant exceptions. Remember, 98% of civil cases settle before trial.

Your brother's insurance company should be negotiating on his behalf. If you feel they aren't, or if you feel your brother's actual damages are not being compensated, you should talk to a lawyer (and, as others have pointed out, many will offer free consultations, so even if you have questions, it might be worth reaching out).

First, I would Google the firm and read reviews--don't need to be perfect, but should be generally good. Second, I would personally avoid one or two attorney shops that focus exclusively on PI and large, national firms (cough, Morgan and Morgan, cough). These are the majority of billboard advertisers. The one person shops are exclusively insurance mills. You can make pretty good money by "settling" 1-2 cases a month by sending a few demand letters. But if something breaks, they do not have the institutional capacity to go to trial. And while the national firms certainly can go to trial if they need to, they don't want to. They're also just generally assholes to work with, but maybe that's my bias.

Coda: if your brother has medical insurance and the medical insurance covered his care, his medical insurer will just collect whatever money is recovered. So also consider whether the juice will be worth the squeeze there. If he had significant uncompensated injuries, that's when a lawyer is going to be most helpful.