Bulldogs World

I'm finding out that the dog food we deemed as "perfect" for Fatboy has only one draw back.  It seems that it's causing the need for his anal glands to be expressed.  His stool isn't firm enough to work in that department.  When we switched him to the new brand he had to go in for surgery due to an anal abscess that wouldn't rupture.  I'm seeing again, about one month later, that he is scooting his bottom on the floor like he did prior to the surgery.

My question is, how often do the anal glands need to be expressed?  And should I always allow the vet to do this?  Or is having the groomer do this a safe choice?  Also, wold mixing some of the old food that made him slightly constipated be beneficial in this case?   I'd rather not have to take him in everytime to have the vet do this when the issue can be remedied at home by diet alone (if that's possible).