Okay, so a teaching strategy for developing friendships:
In a small social group or class, put two giant different coloured hula-hoops on the ground (one red and one green). Hold up pictures of favourite topics i.e. food, books, tv programme, activity and get those that like the items to go to the green hula hoop and those that don't to go to the red.
Teaches 'we like the same thing' or 'we don't like the same thing' and the motor movements promotes participation and attending. From there you can indentify common interests of the children and use that to plan and foster the 'shared' interest. Possibly even plan playdates based on the common interests.
(The above is just the first strategy I found on a randomly chosen page).
There are 3 levels of social skills checklists and for each 'skill' you rate whether it can be done at 1:1, group or a natural/spontaneous setting, so you can track progress.
For each level there are 'modules' with module 1 being Joint attention, 2 greetings, 3 social play, 4 self awareness, 5 Conversations, 6 Perpective taking, 7 Critical Thinking skills, 8 Advanced Language, 9 Developing friendships, 10 Community/Home life.
So an example of 'social play' for level 1 would be turn taking for 5 turns, level 2 Follows play and understands games such as who's IT, and level 3 might be awareness of pop culture games/play.
It is probably very basic and obvious to professionals but I always get the feeling that people like to talk a lot about deficit social skills but gloss over what that means or what might be done. Instead preferring to suggest that ds will get them in time by just being amongst his peers and if he doesn't then that will be because of his ASD.