Am following this thread (and other similar themed ones) with interest.....not for me, but to get a gauge on things for DS when ex eventually introduces DS to his new 'girlfriend' aka the OW.
Unfortunately, ex and OW have chosen a fantastic set-up (for themselves of course) in that OW, who is moving to the UK from the US, will be moving in with ex from the day she reaches our sunny shores. They will only have spent sporadic periods of time together since meeting last Oct when ex was in the US for work meeting. Longest period of time ex has spent with her was 2.5 weeks when she came here for a visit/holiday back in April. Before then, they had spent no more than 6 nights in a row together either here or in the US (rest of their relationship has been built on skype/emails etc). Like PP, I am very overprotective of DS, though he is a warm, sunny and friendly child. I know legally that ex can dump current OW and meet his next soulmate on his flight home (he is in the US now on work/holiday with her) and introduce DS within hours, though hoping to appeal to his moral side in terms of looking at this from DS' perspective. OW moves here in July and as DS has just started sleeping over at ex's house for a couple of nights a week, I am hoping that ex will see reason and NOT insist on DS keeping this routine up initially with OW in house. Really not sure how to manage this!!! Esp as I know that I am not wholly in control of things. Is it reasonable for me to ask ex to have the usual initial meeting in a play place one weekend and then build it up? Ex is mad keen for them to meet as it fits into his 'everything settled, lets get on with life' approach. But knowing ex, he has a tendency to 'push' DS onto other people and can easily imagine in 'forcing' DS to be a certain way whilst in OWs company (in fact, it was a close friend who raised this with me as she knows what ex is like!). This was never done in a malicious way when we were together; more like if DS was shying away from a friend at say a BBQ, ex would keep encouraging DS to go up to said friend/trying to get him interacting with friend iyswim.....??
I think ex is still in that honeymoon phase so probably not that fussed about DS' perspective...?
I would love to suggest that he cant have DS over until he has been living with this woman for at least 6 months, but I know he will start banging on about his rights and to him, he considers that he 'knows' this girl and that they have been together since Oct, albeit long distance and the glaring fact that they have never lived together, unless you count that 2.5 weeks in April?
My suggestion to him would be: that he can see DS as per usual but introduction to OW would have to be slow, so one week at a play place, another week for play in park, then lunch etc. Ex could still see DS but without OW (Please dont flame me! I know that all I can do is reason with him... 
). Besides, shouldn't he be aiming to spend some time with DS alone for 1:1 time anyway?