delayed milestones in my child

Dear Doctors , I shall be very thankful to u if u could help me regarding my son.
He is 5 and a half years old. Is the second baby amongst twins. The first one expired soon after birth. He was greatly underweight , had jaundice. He had severe congestion after one month of his birth. Had turned blue at that moment and it was as if we had almost lost him.He recovered after 15 days and then his delayed milestones began. He could hold his head after 6 months. Sat after 1 year, walked at 3 years , spoke at 4 and a half years though he used to babble a lot since 2 years.The only thing which seemed to be on time was his teething.He has hardly been sick ever , eats everything, chews well, has a good appetite, is a happy child, never cries, plays a lot, is very active. We helped him with physiotherapy and speech therapy since a long time.Now too he has a special trainer, a speech therapist, a physiotherapist
and an occupational therapist at the school he goes to.His looks are totally of a normal child .
My concerns today are -
1.He still cannot go up and come down the steps on his own. Is scared though he has started doing a bit better now.[ with physiotherapy]
2. He has a peculiar habit of keeping his one hand on the cheek and biting the cheek with his teeth as if it is a kind of a support. This looks not so normal.
3.He uses both his hands together to so some action, like saying bye.
4.He still babbles words in his excitement [very loud] that have no meaning which is very irritating and shows his signs of being different though he has started speaking [ with not much clarity].
5.He has a peculiar habit of jumping with a perticular hand movement [ both hands] when he is excited which again is not that normal.
6. He still cannot run.
7. He cannot write yet cos of has motor skills being weak though he memory is very sharp and learns everything by heart.
8. He is very thin and small and does not look his age. He looks around 4 years of age.
I have penned down all that came to my mind at this moment. I shall give any other information required But please i would be very thankful if any of u could suggest a remedy that could speed up things and my child could lead a better and a more normal life.