As the thread seems to have been revived, I'll give an update. :-)
Ds's behaviour has improved a lot. It seems to be since my mum has gone back home. She was staying with us for two months and I think the extra attention didn't do him any favours. He still has plenty of 'moments' but is much improved.
I had however already sought a referral to a paediatrician and after checking him out and giving him the all clear both medically and developmentally, referred him to a psychologist for social anxiety. We saw her for the first time today. He's scoring high for social anxiety, but thankfully low for general anxiety and everything else, so he's starting some group sessions with other kids soon. I hope this will help him to feel more comfortable talking to adults and getting involved in group.activities at kindy, before he starts prep. :-)