Hi Debbie -
Sorry to hear about Ollie - but I do so agree with Carol.
I think your veterinarian is giving you the worst possible scenario - when in fact some of these dogs can and do go on for years with appropriate medication.
As a vet. I saw and treated many many of these cases - and I never predicted the outcome - as it could be so varied - and that was before many of the more advanced drugs we have these days. I know one 12 year old Cavalier now whose heart is appallling - sounds like a washing machine - but he has very few symptoms and is on minimum medication and he has been like this for 5 years at least.
I am not saying your little one will last that long - but take each day as it comes, enjoy Ollie and you may be surprised.
Obvious things to do - keep his weight down, avoid him being out in very hot weather - don't let him get overtired.
Supplements - well you can talk about them for ever , but most important things I have mentioned - keep weight good and avoid stress and heat for him.
Best wishes
Pat Morris B.V.M.&.S. M.R.C.V.S.