my oh doesnt have issues with previous partner cheating, however he too became distant and grumpy after the reality of my pregnancy kicked in (one we'd both planned for). one night he went to his mates house and didnt come back for 2 days over something stupid. I still feel angry at the way i was treated; at my most vulnerable time i never knew if my oh really wanted the baby. he always told me it was my hormones which made me so angry - it wasnt.
I can tell you though that since we've had LO it has all melted away - he adores him. I think he was just immature and scared tbh - no excuse i know.
I think you need to say to him how sad and vulnerable you are - this is the biggest time of your life emotionally, physically - in every sense and to be accused of cheated putely because of his past - which you cant control - is devastating for you, and your relationship. He needs to grow up! DNA tests are not safe prior to both - you're hardly going to risk miscarriage to have an amnio! say this to him and hopefully he will realise how selfish he's being.
Dont ban him from the scan - i know you're angry but his relationship with his child needs building on, it might be the bonding moment he needs. Big hugs and please let us know what happens.
xxx