Constipated dog.

We have a large, 45kg, dog, mixed breed.He's five and a half years old. He is straining when he goes to the toilet. He eventually produces something but they small pieces. He seems to be in some discomfort after going to the toilet but after 10 mins or so this settles down and he's fine. His stomach isn't tender when we touch it and he doesn't seem bloated. He's eating normally. We actually live in Bulgaria where the vetenarian service isn't so good. We feed him on biscuits with chicken liver, chicken hearts or chicken necks, all well cooked first, added for 2 days and then tuna in oil with pasta on the third day as he is also suffering a bit with his joints and we feel the oil may help keep him moving. In the morning he has toast with sunflower marg on it and during the last few weeks when he's been suffering I have made him a porrige from bran. He doesn't seem to be suffering every day just every now and then. The vet in town has offered us some tablets that will make him go to the toilet but I'm not sure if thats the correct solution. Please can you offer any advice. We feel sometimes that the kibble we buy here is maybe sub standard, the better vets tell us not to believe what we read on the bag when it comes to contents and in the small town where we live it's sometimes difficult to buy a well known brand, and even then it may be a fake. This is the reason I add the chicken liver etc as an extra bit of goodness. We also have his brother , slightly different combination of dog but he can eat anything and seems fine. They were strays we picked up locally as pups so can't really give you much of an idea as to the original breeds.We believe they originate from some kind of hunting/gun dogs.
Any advice would be welcome