Welcome!
My first thought was that if your DS is already at first grade level then you have the issue that by the time he can officially start school he is likely to be several grades ahead of his age peers. While you say homeschooling is not possible you will need to seriously consider all your options for your son's schooling plans. He is likely to need an individualized path. Start looking now at what exists in your area. Unfortunately, the more divergent your child is from the usual then the more specialized your approach may need to be. Are there specialized gifted schools or programs? They are not necessarily the answer as they vary in their ability to truly cater for highly gifted children but start looking as two years will fly by.
For now, as Portia says, let him pursue his interests. Make friends with your local library. If you have a toy library that could be good as he is likely to burn through toys. If he is a builder find construction toys which you feel are appropriate for his age and skills, source documentaries of topics of interest, museum visits, musical activities etc.
Meanwhile, read through the forum discussions on educational options, become familiar with the Hoagies website (
http://www.hoagiesgifted.org), read over the information on the Davidson database (
http://www.davidsongifted.org/db/Article/Davidson_Database___Overview_415.aspx), enjoy your child and hang on for the ride.
