Just simple stuff. The syntax of your sql statement is wrong. The value coming from the combo control is alpha, so when you concatenate it into the string you have to include the opening and closing quotes so that Access knows to treat it as alpha and not attempt to treat it as a number. Thus, it should be:
The form referencing was incorrect as you suspected:
="SELECT execs.CUSTOMER, execs.[salesexec]
WHERE ((execs.CUSTOMER)='" & [Forms]![Filter]![Combo] & "');"
Finally, I don't know if others will agree with me but unfortunately, Microsoft have taught us bad habits by allowing spaces in file names. My early computing was done in Unix and MSDOS, both treating spaces in file names as signifying the end of one name and the start of another. Consequently, the habit of spaces in filenames follows through into code and databases as I see it all the time with younger programmers that I teach; I never use spaces in object names or references. If I feel the urge to use a space, I use the underscore. I personally don't believe that it is good practice and I'm sure most seasoned programmers (I don't include myself in that category by the way - I'm still learning as far as I'm concerned) will agree with me. So before you use the code above, note that I have changed the object names to remove them for my testing.
'Forms!sfExecs.RecordSource = Forms!Filter!SourceCode
Me.sfExecs.Form.RecordSource = Forms!Filter!SourceCode
All in all, you were pretty close - its always the little things that cause the breakages! And the frustration!
At the risk of incurring the wrath of the moderators on this forum, I am attaching a doc file which you might find useful. It contains examples of the syntax for referencing forms, subforms and the controls/properties for them. I've found it useful as a reminder as I am not writing code every day so it is not second nature to me. I hope it will be of use to me. I can't take credit for it - I found it on a website some years ago I think written by a guy called Keri Hardwick. Anyway, I have always been grateful for his contibution to my efforts.