If actual field-level validation of that type needs to be done, then you possibly shouldn’t be using a telnet (nor similar) terminal interface. By definition, 5250 terminals work in ‘block’ mode. The entire screen is processed whenever communications happen between the workstation and the program.
You can create programming and display file definitions that more or less function as you describe, but it becomes very complicated after just a couple fields are involved.
Further, if the user simply moves around with a basic cursor movement (arrow) key, you would have no control at all. Your program won’t be notified.
Because cursor movement can be done to any position on the screen without notification to your program, things can be unpredictable.
In short, if you want field-level validation, you should consider a GUI front end.
Please provide a basic outline of what you see your proposed screen layout looking like. Especially, how many input-capable fields will you have and how many should have ‘field-level’ validation? It might also be useful if you describe the validation that should be done. There might be alternative screen-handling methods that would fit.