Hi
Don't know your ds's age or much about him, so am sharing this in a generic sense.
In our secondary unit for kids with HFA, I use a therapy technique adapted from Michelle Garcia Winner (www.socialthinking.com).
Essentially, each student has a number of coloured squares of paper. Each student has their own colour that identifies them. In conversation, each new turn/topic change reqires a piece of paper to be placed on the table in front of the students. As you can imagine, initially, this has to be demonstrated/fully prompted by adults (we do a 2:3 ratio of adults/kids for this group initially).
Topics are generated by the students and written on cards and put into a box, then chosen at random. During baseline assessment on this activity, we make sure we see each student in conversation on a topic of their own choosing and in conversation on topics of others choosing.
Initially, adults model turn-taking, showing that generally, in a five minute conversation, everyone has a turn so that a range of colours are used.
What tends to happen at baseline is that topics chosen by a student (particularly those relating to their special interests) will tend to involve that student talking a lot about their interest with other students either remaining silent or interrupting/talking across them. However, it's really visually obvious if, say, John uses up all 30 of his red cards while no one else has gotten a word in edgeways or if he continues on in a line while two other students branch off and talk about something else, or branch off individuall to both talk about their own interests.
At the end of the conversation (typically about 5 minutes), the students look at the visual schematic of the conversation and we ask what's happened. This becomes the platform for talking about how to improve on the interaction.
I feel I'm not explaining this very well, but when I began this with a group of Year 9/10's last year, they very very quickly identified when people were talking too much on one topic, not listening, interrupting etc and showed beginning signs of 'real time' awareness e.g. one student said "oops, miss, I was about to crash into him there because he kept taking his turns and I thought it was my turn".
At the very, very basic level, the idea is just to develop an awareness of the need for equal/balanced turns in a structured group on a specified topic using visual and verbal cues, as evidenced by a) understanding and being able to explain why something didn't quite work or b) as above, identifying that you should do something differently.
When a few sessions have been done, then we look at individual targets within the group e.g. for x to increase turns on a topic that is not of interest by asking questions or for y to decrease turns on a topic of his own interest by making two comments and then asking a simple question of another group member e.g. 'what do you think'. Those were examples of high level targets for students who "got it", quickly.. another might have a very simple target to say one thing about a topic or, as was the case with one student, stay with the group and not wander off.
It can be very, very highly structured or quite fluid, depending on the students.. and the aim, really, is to increase awareness and some specific, set behaviours within a controlled environment. It is easily adapted depending on level e.g. for less conversational children, simply taking a turn in a circle to comment on something e.g. a funny object pulled out of a bag, or a drawing. We find text cues work well.
I have also used a similar yet different strategy to promote pairs of students to ask questions/answer questions by giving each two piles of different coloured cards e.g. blue and orange - blue with a question mark and orange with a speech bubble - showing them the "pattern" of what is expected e.g. blue, orange, blue, orange, blue, orange (two columns, side by side, works well). Again, with younger children, this is kept ultra-controlled so it can be used, say, alongside "Guess who" or another barrier game.. progressing (slowly) towards talking on a topic. It is highly dependent on the child how much structure needs to be put in place. I see the structure, in this type of activity, as a way of freeing up processing time to allow them to focus on analysing a real-time conversation, with cues gradually being removed.
Have no idea if any of this makes sense!