If a person goes to the police to report himself or herself being victimized, whether it's sexual assault, domestic violence, threatening behavior by someone or some other situation, there can be this awkward dynamic in which the police officer is seemingly telling the steps involved in making a report but is also subtly (or not so subtly) trying to discourage the person from making the formal report. I think the police officer does this to reduce case load, and also probably because he or she does want to be criticized by fellow officers for taking a case unlikely to result in prosecution or conviction.
Now, if the person who was victimized instead goes to the police with a friend or family member acting as a low-key advocate, that is likely to change the dynamic. The police officer is considerably more likely to take it seriously and considerably more likely to be professional in a good way.
This is not a hundred percent of course. But I do think it substantially puts the odds more in the person's favor. (If a friend or family member is not available, I think taking in a written one-page or one half page summary also increase the odds, but probably not as much.)
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And so with your brother, you might be able to act as his advocate if it comes to it, if you're his best possibility, even if you're not yet 18. Of course it will be better if his advocate is an adult. And probably better still if his advocate is an older family member (unfair, but probably most likely the case).
The first step is to get the police officer's attention and get him or her to take the situation seriously. Brother cried for hours, said "bathroom," etc.
And once you've said enough, or if your social read of the situation is that it's obvious that the police officer is taking it seriously, then ask for what you want. Maybe something like, Even if an officer just goes to the restaurant and asks questions, even if you can't eventually make a case, just seriously asking questions, sends a message that there are consequences to messing with my brother. And hopefully, will make future abuse less likely.
Best wishes for a difficult situation.