I understand that you are frustrated with the situation but it looks like your dog's mass was located in a tricky spot. It is very close to if not directly over the hip joint from what I can see in your pictures. This is one of the hardest places to get sutures to hold. You essentially have to limit the dog's movements until the sutures are removed. Do not allow him to stretch that back leg too much: no jumping up on you, no jumping onto beds or couches or into cars/trucks and anything else that is going to extend the hip joint and pull at the skin in that area. You may have noticed that the sutures did in fact look loose at first, I can't say for sure, but I believe that is commonly done over joint areas to allow some leeway for the natural tension that is put on the skin in that area.
I hope that some of this information helps! And good luck!