B.H.
My son also has long, flat, wide feet & I was under the impression from our pediatrician and ECI therapists (for other issues) that my son's & daughter's arches would develop over time.
http://www.bio-medicine.org/medicine-dictionary/Flat_feet/
They've all recommended Stride Rite shoes for both my children even though they have very different feet. DD has long but narrow feet & ankles. They've also said that it's important to make sure that as their feet are developing to put them in flexible-soled, well-fitting shoes.
I do buy Stride Rite - I hit the sales and lately, have been buying on ebay. Have bid & won for less than $10 on XW, EUC shoes for DS.