I suppose (but quite obviously have no idea) that he thought it would simply be easier to report the transaction on his own return (1040) than bother with a 1041. The is the case, of course, but it was not exactly the proper way to handle this scenario.
There is most likely not much you can do at this point since (a) you cannot force your brother to amend his own income tax return and, (b) you are not the proper party to file the 1041 (which would entail paying additional tax on top of the capital gains tax that your brother already paid).
The approval letter for the EIN is not what I had in mind when you indicated a letter was received. I initially thought it was a letter inquiring as whether a 1041 had been filed and if not, why not (although it may be a bit early in the 2023 tax year for that letter to be generated).
