Shehz you have made your chocie and that is great. For the record I would go for the second scan if there were no issues re baby's health with a second scan.
However there are potential issues and you should tell your dh.
www.nhs.uk/news/pregnancy-and-child/warning-over-souvenir-baby-scans/
I would just do all you can to support your dh. I totally understand his concerns, and your concerns. I think he is getting a rather hard time here. Yes, you are carrying the baby, but it is his baby too and his worries re his awful over-bearing mother must be very difficult to cope with.
I would also strongly suggest you and your dh work out your strategy for coping with his mum regardless or whether baby is a boy or girl.
Re "Seems to mean the world to DH as he was going through a rough patch with work/parents and wants to bond properly with the baby and now it seems to annoy him that he can't be sure if its a he or she."
Encourage your dh to sort out this issue now. You are going to need him, and him fully aware and ready to be there for baby and you. Of course he is still a son, and still has a job etc but you and baby come first.
Congratulations
boy or girl/girl or boy, this baby will rock your world! XX