I don't consider those CEC levels as being "low".  We deal with CEC levels at that level all the times.  He should have no problem retaining potassium with those CEC levels, if he is getting enough cow manure applied.  

My question is... how much cow manure is being applied per acre?  

Those soil O.M. levels are lower than i typically see.  I would working on building O.M. levels.  I do agree that the sugar beets are hard on the O.M. levels, and probably is a detriment to building soil K levels as well.

I'd take a field that you are going to apply cow manure to.  Pull soil samples before and after applying manure.  Then, on part of the field, I would at least double the rate of cow manure that you normally do, on that part.  Apply the standard manure rate on the rest of the field.  Pull the second soil sample a couple of weeks after applying manure. Look at your soil test K levels and see how much the soil K levels increases with  higher rates of manure. 

edit: Do you know what your soil type is?  Is it a sandy loam? sandy loam? silt loam? etc, etc.   

Also, what do you with your fields after wheat is harvested?    I believe I saw wheat is in your rotation.  So, how does your rotation work?