While I don't have the Down's Syndrome to contend with (and I'm sure that can make it a lot more difficult at times), I do understand the worries over a child not eating enough. The best advice I was given was that they will eat when they get hungry enough. However, I still worried over their vitamin intake. My suggestion hinges on whether or not he would actually drink either of the following... Pediasure (which can be a little expensive) and/or Carnation Instant Breakfast. If he will drink flavored milk in the morning, he will probably drink one of the flavors of Carnation Instand Breakfast which is loaded with necessary vitamins and minerals as well as having the calories he would need in the morning. And you never know, sometimes getting those calories in over the course of a few days can start a person to want to eat in the morning.
It can't hurt to try. If he is lactose intolerant, try adding it to Soy Milk instead or the Pediasure. My kids like the taste of the Carnation Instant Breakfast, though. Best of luck.