Sorry it's taken me so long to get back to this. Mad couple of days here.
You mentioned that you're concerned that ds might feel patronised. I think this would justify some achievement stories before you start on behavioural modification ones. Most children like being praised and their successes acknowledged (hope yours does too!) and this would be a good way of getting him used to the concept of social stories without him feeling undermined or threatened in any way.
Pick anything that he has done well. I used to write them for ds's karate gradings, swim levels etc but also for routine things like putting his laundry in the basket, clearing his plate etc - if we needed a positive, I could always find something to write about. It sounds a bit faffy but it can be very effective if done right.
Overall, social stories are meant to be positive and encouraging, so no use of negatives to point out what he's getting wrong. Its' all about phrasing the text to direct him towards the 'right' thing to do, so I'd be working on the benefits of not running away, rather than commenting on what a pain it is when he does. Perhaps something like:
"After school we need to get home quickly so that .
I like to do when I get home.
When it is time to leave school, I will try to get my things together and coat on and be ready for Mum picking me up. Then we can go home quickly and I will be able to ."
Obviously develop that to relate to your individual circumstances.
A choice card can narrow this down into good and bad consequences, BUT I would use them separately and try the social story for a few days first. You might not need the choice card at all if the social story has the effect you want. Remember the social story should always direct towards the positive and the choice card can introduce less favourable outcomes if the message doesn't get across.
I'll also add (for the benefit of other people) that I'm not advising choice cards for every situation, or even for every child. I devised them simply because there were so many situations for MY son that social stories didn't work for - ds needed to know what the negative consequences would be (it's just how he was) so we had to have a tool that would cover that.
And seriously - take your time
. It's far better to spend a few weeks thinking it through properly and have more chance of success than go bowling in with something that's not properly prepared and risk failure.
HTH