Titty, You said, "I don't think I'd be having unnecessary scans. They can affect baby."
In what way can they "affect [the] baby"? If you are going to scaremonger, can you at least expand on your statements?
There's some really, really weak evidence that repeated ultrasound can make a child more prone to lefthandedness and that seems to be it. (The research was done by one of my previous fetal medicine consultants, which is why I happen to know about it: www.ncbi.nlm.nih.gov/pmc/articles/PMC1678377/pdf/bmj00030-0021.pdf)
Other than that, there's nothing that concerning, beyond "we don't really know so best not to do it too often", which doesn't really work for those of us who have to have an awful lot more scans than most because of previous dead babies or complications. I'm willing to 'risk' a lefthanded baby. 
(DH actually really disappointed that none of the children is lefthanded so far as he is!)
As for the OP's DH, I'd see his request in a more generous light. He sounds as if he is dealing with significant anxiety and this (however unreasonably) seems to him a way to 'fix' it. Having dealt with anxiety myself, my guess is that he would then not trust that scan either or find some other issue to focus on.