I would never refer to a boy as slight or substantial. I would say they refer more to an amount.

I'm not so sure you can. I honestly wouldn't understand either of them.

Remember, the 'more advanced/more complicated' word isn't necessarily better. Ask yourself if your reader will understand it better. That said, there might be times when you would want to use these. Slight = Thin, Substantianal = heavy. They don't have to do with age. They shouldn't necessarily be the default choice though.

Usually, "small boy" refers to a boy who is very young: "I remember exactly how my grandfather smelled, even though he died while I was still just a small boy." We also say "little boy" to mean a very young boy. "Small boy" and "little boy" do not mean anything about size, height or weight. They mean only that the boy was a young age, perhaps 4 to 7 years old. If you mean that a boy is thinner than other boys his age, you would need to choose different words, e.g.: "He was a skinny kid." "He was a thin boy." And, yes, you could then say "He was a slight boy." But this sounds a bit old-fashioned (i.e., something that people might have said 50 or more years ago). If you mean that the boy was shorter than other boys his age, then you would say "a short boy" or "not a very tall boy" or "a shrimp" (though this is very informal and a bit unkind). Usually "a big boy" is used in talking to a young boy who is acting mature or who we would like to encourage to behave well. A mother may say to her five-year-old son, "Now, be a big boy and finish eating your vegetables." This means "behave like older boys behave." If you mean that a boy is heavier than other boys his age, you might say something like this: "He was overweight." "He was a plump boy." "He was a hefty boy." If you mean that he is stronger and that his body is bigger than other boys his age, then you might say something like this: "He was a stocky boy." "He was a sturdy boy." "He was a hulk of a boy." If you mean he was either taller or fatter than other boys and you wanted to be kind, you could say "He was a large boy." And, yes, you can simply say "big boy" to mean he is physically big compared to other boys his age. But you will need more context to make this clear. Imagine a story set in the year 1901. A woman and her 12-year-old son have just moved into a small town. A farmer might say to the woman, "That's quite a big boy you've got there, miss. I wouldn't mind his help out on my farm. I can pay."

Neither of those sound so good. This topic is about collocations. In this case, the combinations that you used are not collocations, meaning that even though the terms you used make sense logically, the words don't sound good together.