Another dr. appt concern

So it just popped into my head today for some reason a part of the conversation we had with the Dr. about Jonah and feedings. He said his weight was fine but that because he was not tolerating any formulas so far, and the fact that he was premature,we should wait until he is at least 6 months to introduce any solids. I was planning on doing this anyways but anyone else deal with this? If so, what foods did you start with? We have had such problems with formula that he is only getting breastmilk.