Forget previous suggestions. Here is another, simpler, approach to convert the month name to month number:
Month([Month] & "/1/" & [Year])
Use the expression in query to construct a field MonthNum.
To find the latest month for a year:
DMax("MonthNum","queryname","[Year]=" & [Year])
The DMax can be criteria under the MonthNum field.
I don't have your file with me now to test this. Maybe this weekend. Let me know if you figure it out before then.
BTW, Month and Year are reserved words. Best not to use reserved words as field names. If you do, must enclose in []. Same applies if spaces or special characters are used.