Even the AAP says very clearly that solids during the first year are "supplemental" whether a baby is breastfed or formula-fed. What your doctor is advising is really poor advice.
Nurse on demand, continue offering solids as you have been, and don't worry about this!

If you feel you need to revisit this with your doctor, look for the AAP guidelines on solid introduction and bring those in to him. Perhaps if he is reminded of the actual guidelines, he'll change his advice to mothers (not every mom would question his 'advice,' and look for independent information about this) -- and, if he's not receptive, it's a good sign that it might be time to look for a different doctor.
Kellymom may have links on her site directly to the AAP statement on the introduction of solids/solids during the first year.