I would try not to think the worst just yet... I know it's hard not to, especially with the nurses saying that... I'm surprised they acted that way, as if it's already time to mourn... On the bright side, isn't it a bit of a reassurance to them that his heartbeat went up to 158? If it would have stayed at 100 for a long period of time that would be worrisome, but I would think it's a good sign that it did go up when he moved. He could have just been resting during the time of the ultrasound, how long were they checking? I'm also surprised they didn't keep you there longer to monitor the heartbeat and his movement... or is it too early? Can they do non-stress tests this early? What about an anatomy scan? I know those aren't usually done until 18 weeks, but if they did one they could see if there are any abnormalities causing the slow heartrate.
I'll be praying for you, but hoping for the best... just try to not think the worst. The slow heartbeat could be the result of many different things. Even if it is a heart defect or some other birth defect, many things are treatable, even in utero.
Think of all the obstacles that had to happen for your rainbow baby to get here. He is a strong one and I just know that he will pull through this!