You should talk to your doctor about what his needs are ex. like if he is gaining the correct amount of weight and should also look into a therepist to help with cognative and physical help me grow is a great place that will help you through all the things your infant will need to help with his growth and development.

I have a 7 month old son with DS, and our first call was to the school district. I'm not sure where you live, but here in Minnesota a Down syndrome diagnosis qualifies kids for Early Intervention. A teacher comes to our house once a week, and my son also has a PT and OT assigned to him through the school district. They work with the child, give us ideas on how to encourage his development, and are great resources if you have questions.

I would also make sure you have a doctor who is familiar/comfortable working with kids with Downs, as there are different tests/specialists needed for kids with Downs. Our local Children's Hospital has a Down syndrome clinic where all the specialists are in one place and they will coordinate everything you need. Our son was blessed with perfect health and our pediatrician is comfortable treating him, so we opted out of the Down syndrome clinic option. 

Most of all, just love him to pieces and know that you are not alone with your fears, questions, range of emotions, etc.