My 3 1/2 month old baby wont track...?

hi there! I am a mother so my second son and he is 3/12 months old! ;-) I took him to Pediatrcian concerned that he does not have great eye contact and does not track... I took him to the eye dr and there is nothing worng w his eyes... he smiles back when he does look at me and coos as well and he is overall and GREAT baby sleeps all night etc... I asked my dr if i need to be concerned and he said NO just to really work with him showing toys tummy time etc.. and I havent been because i also have a 2 yr old. ;-) of course as a momma i am a little concerned... has anyone else has this and there babys were fine?? any comments would be great with any suggestions..thank you so much