Is there any way you can post pictures? Maybe some of him standing so we can see? In all honesty that food is not a very good food and that may have something to do with it. I know how hard it can be on one income as we are as well but the better the food, the less you will have to feed so that $50 dollar bag of food will last longer than a $20 bag of food. I'm not criticizing you by any means but that may be the issue. Also foods such as that one are full of grains which some mastiffs have a problem with. There's minimal protein from the chicken and most of the food are fillers. It's like feeding cereal all the time. Your pup may not be getting the nutrients he needs. Another thing is that a lot of mastiffs have a problem with chicken as the protein source. Again please do not feel like I am criticizing you, I understand that you are probably doing the best that you can do for your dog and for your family. How are his stools? Does he have any other problems like rashes, itching, hot spots etc? It can be very hard to find a vet who is knowledgeable about giant breeds so either it's time to try a new vet if you aren't comfortable or start doing your own research to bring to the vet. If a dog is a healthy weight you will be able to feel (not see) the last two or so ribs. From above he will look like he has a defined waist and doesn't look like a sausage. Also from the side you will be able to see a tuck under his stomach back towards the groin, his stomach won't go straight across from chest to groin. Mastiffs in general are slow growers. Many do not mature until at least 2 years but often closer to 3 years and sometimes even beyond that. Do you know how big his parents were?
