Hi and welcome,
First, DNA testing can take place at any age and it is the only method that can tell exactly which alpha traits your child carries. A birth electrophoresis would have revealed hemoglobin barts, if it had been done, but this needs to be done at birth. A DNA analysis looking for the traits that each parents carries is the only way to learn what type of alpha thal your son is.
And to add to your understanding of alpha thal, alpha majors cannot survive unless transfusions are done while still in the womb. An alpha major has 4 genes deleted and cannot produce any HbA, or HbF. This is not compatible with life. A 3 gene alpha is considered to have either HbH disease or constant spring but they are not labeled major. Many with constant spring will require transfusions, but their phenotype is closer to that of a beta thal intermedia patient.