wow that's a tough one! i typically wouldn't advocate not telling the truth..but...

if he doesn't ask, don't tell!

perhaps if he asks, you could answer with "you know snu was old, and dogs don't live forever." not actually saying he died of old age.

you have to know one day it will slip out and he'll find out the truth. but it may be easier to bear at a later date. and it's one of those fibs that he'll understand why you told.

like i say, i hate to tell anyone to lie to someone. but if it were me, i can't say that i'd rather know the gruesome details! i'd rather think my dog died a peaceful old-age death.

i would include your younger son in the decision. because you are asking him to be part of the fib as well. i'd definitely explain to him that you and your husband found this to be a hard decision to come to and that you never lie to them. etc. if he is not ok with the lie, then you'll need to tell the painful truth. but i'm sure he'll understand!

you could also choose not to tell him the truth now and purposefully wait six months or so. then sit him down. explain your reason for waiting. no one should have those images in their head around the birth of their child! just another option.

again, i'm very sorry you're having to go through this.

take care!