It's very helpful to hear how other people handle this issue, thank you.
We do use humour to handle it, like you Polter, as DS is now able to laugh at himself (sometimes). But there are other times when he's so passionate and earnest about a subject that I feel it would be mean if I broke the spell by admitting to him that he's going on too much about it. He doesn't have any RL friends to chat to at the moment (he lectures people chats online), so we are his only source of social contact until he goes to his new school (a painfully slow process because of his anxiety). However, I'm aware that we won't help him make friends if he hasn't learned to have a two-way conversation, so I probably need to get over this feeling or I'm not helping him.
zzzzz - I like your description of conversation as 'verbal sharing'. Think I might borrow that to use with DS at an opportune moment.
DS does sometimes realise he's going on about something and asks me if he's boring me, but then - just like OldAntiquity's DS - he just carries on!
I suppose one of the main problems is that DS is very out of practice at being in a social setting (he's been out of school for three years now), so he hasn't had the chance to observe other people conversing, as you have done, Antiquity. I'm hoping being back in a school setting will help him with that.
clanger and bbkl - I really like the idea of using a sitcom as a springboard for talking to DS about this. He used to watch 'The Big Bang Theory' - I wonder if he'd agree to watch it again with me...
streaky - if I pointed out to DS that he won't be popular unless he learns better conversational skills that will be a disincentive! He had a brief flurry of popularity at primary school and found it terribly stressful. He can only cope with one or two friends at a time. However, he might relate to the need to learn conversational skills for work and interviews. The BT DVDs you mention sound interesting, but DS is probably too teenagey and cynical to watch them now. Might have worked a few years ago.
ouryve - this rings so true: 'becomes rather belligerent on his pet hates'. I dread triggering any of DS's pet hates.
Thanks again - good to hear how other people handle this issue.