Concerned about Down syndrome

My baby is 5-1/2 months old (DOB 19 May 2009) and I am concerned about Down syndrome because of an earlier remark by the doctor. My son smiles at us, coos while looking at objects (mainly the fan at the top, a rolling toy on the window, my daughter's big poster photograph), recognizes me, my wife, and my daughter's voice, plays with a rattle or a soft toy (mainly a red dog, red looks like his favorite color) when given in his hand.

But when it is kept a little high on his eyes, he does not try to grasp it which I think he should be doing by now. Secondly, he does not roll over to the side, not even trying to roll over. He holds his head up and I make him lay on my stomach like in a semi-push up position. He even tries to sit up when his head is laid on two to three pillows but not otherwise.

He was a pre-term baby born via C-section due to BP complications in my wife, initially had some respiratory issues and was kept in the ICU for three days (on a ventilator for just around 2 hours) and then discharged and then on returning home developed jaundice and was again hospitalized and given phototherapy.

My main concern is that at the time of his birth, the doctor in charge of the delivery told me that everything was fine with the baby except that one of his ears was a bit lower than the other one (not able to notice it unless looking very carefully) and that this was a symptom of Down syndrome. But as per the doctor himself, he did not have any other symptom. But now since he is not developing, I am quite concerned about the Down syndrome and would like your views.

Any wisdom?