Firstly, i'm really sorry for your loss.
Miscarriage can be caused by an abnormality in the chromosomes passed on from either the male or female so it's possible your boyfriend may be passing on something but only tests would show if this was the case.
I think if it's really bothering you, you should go back to your doctor and ask if the tests can be done now rather than waiting to see if it happens again.
Best of luck!