I have to agree with all the others - please don't feel bad about expressing your emotions over this - it's perfectly legit and it's good for him to learn how to process his emotions. You should feel free to be as honest as possible without scaring him. You both love the dog so much, it's ok. I like the idea of trying to visit the dog if you can - and I disagree with the post who feels the dog isn't scared and doesn't know he's sick - dogs definitely get scared and feeling cared for makes a huge difference. Maybe see if you & your son can take a trip to the vet and bring a T shirt that you are wearing to leave in his crate - something with your smell on it and let your son know that your dog will smell it and be comforted when he misses you.
Best of luck to you & your son and your dog. Congrats on the pregnancy. Please post a "so what happened" so we know the dog is better.
Just read your update - AWESOME!!! that's great news, what a relief for you & your son. Yeah!