Hello,
Here is some information I have located on the common causes of hairloss from the black lab side, and might be considered as a possible cause (even though he is a mixed breed, the lab can be dominant of the mix):
*Labradors have what is called a double coat. This means that they have a soft, downy undercoat and a harder guard coat. These two types of coat help keep the dog warm and dry while swimming in cold waters when retrieving ducks.
Generally Labradors will shed their coat twice a year. This is called "blowing" their coat. They are moderate shedders, not enthusiastic ones such as Alaskan Malamutes or German Shepherd Dogs. There will be a certain amount of hair loss throughout the year, especially in more temperate climates. This varies individually; some Labradors shed less than others, especially if they happen to have an incorrect coat.*
I hope this might help answer some of your questions, take care & God bless!
Please see the following source for additional information:
http://www.bellaonline.com/articles/art6093.asp