My brother is autistic and he never slept very well as a child, still doesn't by all accounts. Have you noticed anything else different about your son, compared with his brother? Is splitting them up into separate rooms possible, at least then your eldest can sleep, even if youngest is still bouncing around.
My twins are still very little, they don't sleep as much as I would like... but hey, since when do babies ever sleep as much as their parents would want. Another 6.5 years and I wonder if one or both of my children will be like your 7 year old, a non/restless sleeper.