The time to train the baby is now. He will never be smaller or more manageable than he is right now. You can do practically everything but ride.
I am expecting my first foal, and I bought the book by Robert Miller, Imprinting the Newborn Foal. It teaches you all about tying, leading, ground manners, etc. Get it for the foal you are expecting, but I bet it will help you with your 7 month old too.
I really want to get a jump start on my foal when he is born, because he/she will be my first one, and I would rather try to get some respect and training into him from the start than wait until he is more unruly. So there is no time like the present to start with your colt.