It sounds like your ex is the father.
If you were dated at 7 wks that is the only one date that really matters. After 12 wks all babies grow at different rates so even though you may measure early you due date is still the same. at my last u/s baby was measuring a week or so early but they will not change my due date. I was dated at 6 wks 5 days so they knew when I got pregnant for sure.
Because they are both in the same month and fairly close together, you are going to have to do the paternity testing to know 100%. Good luck!
Hard to say...especially if you don't know when you ovulated. And ultrasounds can be off. My guess would also be your ex, but without a paternity test there is no for sure answer. I would be honest with both of them, so they know and understand the possibility they may not be the father.
My guess would be your ex :)