I have always, always been totally honest and upfront with my daughter. Since a very young age she has understood the good and bad of life. I was raised sheltered and with blinders and swore to never do that to my child. She knew at a young age how babies were made and why mommy could not make anymore. Some might not agree with my approach but it works for us. As a result I have a very mature, empathetic child who just told me the other day that being popular was about having a good outlook and attitude and not your clothes or hair (did I mention she is 9??) Explain to your son what is going on and let him express his feelings, he may need to even grieve the loss of a potitential sibling. I too never wanted my children to be as far apart as I am with my other siblings (7years) but ironically my children won't be 7 years apart, they will be almost 9. I hope things work out the best for your family.
ps don't know if I would get a puppy to get his mind on other things....but then I do have 3 dogs, 2 cats a ferret and 3 foster kittens.....ok...yeah, I'd get the dog
tish