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I have a male 10 year old English mastiff that is 143 pounds and is not too fat or too thin.

If your English mastiff doesn't look thin then he is fine. He will put on more weight as he gets older since giant breeds don't completely mature until they are 3 years old.

My english mastiff was 99 pounds when he was 12 months. He will be turning 2 years old in September and he currently weighs 150 pounds. Since they are a large breed you want to make sure that you are growing him at a slow and easy pace. They don't stop growing for 3 years so he has plenty of time to gain weight. For the first 1.5 years my mastiff spent his time growing taller, now as he reaches 2 years old and keeps growing he will start to fill out and gain weight that way. I just visited his father and he is 180lbs now at 3 years old and when I first saw him he was much smaller. People who grow their mastiffs too fast will regret it when they are older because they will have hip and joint problems. Your mastiff will grow to the size he was meant to be due to heredity so it's better to grow him slowly. I only know because I worried about the same thing, especially because my mastiff is a picky eater!!!

Dogs mature at different speeds. An English Mastiff will not be fully mature until at least three years of age.

What are you feeding this dog? Is he getting moderate, controlled exercise that doesn't strain his young bones and joints? How large are his parents? Has he been fixed?

I like the look of a english mastiff. They are so cute.

you can google AKC and find out all kinds of information one breeds...but if all the ribs are sticking out and sharp shoulder blades and hip bones then i would say yes he sound underweight