If the facts are as you describe them there is no prospect of a successful prosecution for ABH. There has to be some proof of bodily harm (which it seems there is not) and also some proof of intent (which again seems to be lacking).
ABH is a serious offence for which the sentencing guidelines show a starting point of custody. One would have to question why the police should see fit to issue a caution for such an offence. The answer is probably that the CPS would not sanction such �over charging� (if they would sanction any charge at all) and it is a way for the police to show a serious crime cleared up successfully.
Accepting a caution is an easy way for the police to deal with this alleged crime. They have to do no investigation or case preparation and it shows as a �solved crime� on their statistics. For your brother the consequences are a little less straightforward. The matter will show on any criminal record check he has done and this may harm his future employment or travel prospects. More importantly he will be admitting to a crime which he did not commit.
Again, going on the facts as you describe them, there seems little evidence to support any criminal proceedings. Your brother should refuse to accept the caution but continue to cooperate with the police in their enquiries. He should also take proper legal advice at an early stage so that he can examine the evidence which is held against him before deciding how to proceed.