if he is overweight then he doesnt need any hard feed, just a handful of chaff with any supplements in tha he gets will suffice as long as he is getting enough grass/hay.
One of mine is coming up 19 and lives on fresh air, he only gets what Ive just listed.
regarding an underweight horse, its more about what feed you give them than how much
dont ever apologise in your subject title for wanting help, its the whole point of the forum as opposed to pointless threads about nothing (which I do enjoy reading of course, especially when bored at work), we can all learn from it. good luck.