dtexec returns immediately without any messages
You are almost there, the syntax you are using is correct by you cannot use an abbreviation /Val. From the official documentation, you can see that:

/Rep[orting] level [;event_guid_or_name[;event_guid_or_name[...]]
/Pack[age] PackageName: (Optional). Specifies the package that is executed.
/Rep[orting] level [;event_guid_or_name[;event_guid_or_name[...]]
/Proj[ect] ProjectFile: (Optional). Specifies the project from which to retrieve the package that is executed.
Validation and execution phase: The package is run, or validated without running if the /VALIDATE option was specified.

Also note that when executing package from a project don't add the package extension .dtsx
Based on that try to use the following command:
dtexec /Proj my.ispac /Pack my  /Rep V  /VALIDATE