
yes, my username is a givaway I have a Mastiff :) I also changed my pups (Buster) food because he had problems with his front legs - he had weak pasterns & his feet were turning out - rectified by the food change :)
When Buster was weeing loads I was worried that he had some sort of bladder problem as he would just wee wherever he was standing and would even wee in his bed, he didn't seem to know he needed to go until it was happening and on a couple of occasions he even weed while walking around and didn't appear to notice he was doing it :(
It is possible that the bladder takes longer to mature than normal, in which case it can result in bed wetting & weeing when & where they shouldn't, but this is something they can outgrow. It' also possible that your pup has some sort of medical problem like an infection (I'm not sugesting he has a prob like this just a
possible cause)or that he is just one of the pups that take a little longer to catch on.
I don't think you're over-reacting atall, especially when the poor little guy has had such a rough start in life :( Any worries, no matter how small or silly sounding are always worth getting checked out by the vet, the vets dont think caution is silly they are happy people take such a caring approach to thier pets health.
Good luck at the vets :)