How do I make my autistic son who has an urge to always keep moving learn to focus?

You most likely cannot. At five, the autistic child rarely has the self-control, due to his differently-experienced relationship with the world around him-  to "sit still and focus".  That's one of the frustrating experiences parents have with such a differently abled kid. He's FIVE.  Ask any kindergarten teacher, it can be a constant battle to get a not-autistic kid to sit still and focus-  especially if the focus is on something which does not interest him.  The difference is, when it's the differently-abled kid, everyone understands why, and when it is the not-autistic kid, everyone labels it a behaviour or development problem.  He's FIVE.  Give him time to be FIVE. I find the idea of giving the child some other way of being in motion while still sitting relatively quietly will probably end in frustration.  He's FIVE.  He's AUTISTIC.  If you happen to stumble on the one activity (like reading, drawing, taking apart the alarm clock) which engages his attention to the point he can sit still, it is doubtful he will then be able to split his concentration long enough to then "focus" on whatever it is you want him to do.  Autism means the child will react to and see and interact with, his world far, far differently than "normal"  (<---stupid word) children. When you, as a parent, have expectations which are far, far beyond his capabilities, you set yourself up for failure;  be careful not to push that failure onto him.

There are lots of people (some of them not autistic) who focus better while moving. My 11 year old son is NT, but he likes to move while reading or talking. He says it helps him concentrate.

I agree with Clare. I don't work with autistic kids, and at that age, I was hyperlexic and would sit still if I had a book or a sketchpad.