
Have to agree, the reason that the hip scheme has the lowest age set at 1 year is for the bones to become formed. In USA they won't give a final analysis until two, but will give a preliminary opinion at 1 year.
If he is not exhibiting any pain, then perhaps his developing muscles will keep him this way. Many severely dysplastic dogs cope with the condition if their muscles are kept firm, byt frequent short amounts of steady excersise. For example 4 fifteen minute walks steadily on leadinstead of an hour running around.
If your are having second thoughts or doubts, I would ask for a second opinion. You are entitled to one, and the vets shouldn't take umbrage, as this is a serious ireversible step.