H.,
What a scary time for both of you. I would just try to be very positive and upbeat around him. The fear and worry can do more damage than he already has (if he even has any). I'm so glad these other two people have already written a response to you, because they had great input (especially the one about the 68 year old who had a heart attack at 31).
Do you feel like he has a really good doctor? Is this man a heart specialist? It could be worth getting a second opinion....don't know if that should be done before or after the stress test, but I would lean towards trying to get some kind of second consultation before, if possible. Meanwhile, follow whatever guidelines the doctor gave regarding diet and exercise. On the morbid side, you might want to begin talking with him about alternate plans if anything terrible happened (such as, making sure you understand the financial picture, get a living will in place, etc), but I would confine that kind of discussion to just certain time periods (let's talk about this today right after dinner, but only for one hour). That way, you don't stay focused on the negative, but you do realistically plan for the potentials.
The last thing I would say is really the most important, in my mind: are you and your husband believers? You would want to make sure he really has accepted Christ as his Saviour and then just encourage him to trust God and be at peace. Our life and times are truly in His hands, and we can sleep peacefully at night knowing that He has control over all the details that none of us can control!
I wish you, hubby, and Tucker the best possible outcome of this situation, and I will be saying a prayer for all of you later this evening.