Hi and welcome
HUGS
I am very sorry that you are having a very ill piggy on your hands.
Your vets are actually not that far apart in their diagnosis. They are both correctly picking up on the fluid build up in the body cavity, which can be either caused by a failing heart or by an internal mass (usually a tumour of some sort) pushing on the heart and other organs - but it can be often not all that easy to say whether the heart is OK (especially for a vet not experienced with very small animals and depending on how sensitive their stethoscope is) or not. Internal masses are usually masked by the fluid build-up so a scan won't work. Vets often have to make a guess at that stage because it is at the limit of their diagnostic ability and instrumentation.
A low body temperature in a warm room points towards a decrease of the blood circulation. It is a more worrying sign. You were right to see immediate vet care. It sounds like your boys is somewhat borderline at the moment.
Please step in with feeding support asap. Make sure that you keep your boy warm but not hot and in a way that he can move away from any source of heat.
You will find practical advice for feeding care in these links here, including guidance when you should stop feeding in the first link:
All About Syringe Feeding and Medicating Guinea Pigs with Videos and Pictures
How to Improvise Feeding Support in an Emergency
Looking After Guinea Pigs With Limited or No Mobility (the practical tips in this guide also apply for very ill piggies not moving around much)
Wiebke's Guide to Tummy Trouble (see especially the chapter on secondary bloat)
I am never happy to link in this guide here but it takes you through everything, including when the line of no return is being crossed and what to do if your piggy is dying or when they need pts (putting to sleep)/euthanasia step by step whenever you need it.
A Practical and Sensitive Guide to Dying, Terminal Illness and Euthanasia in Guinea Pigs
We cannot tell you whether your piggy is dying off the cuff but they are seriously ill and may not make it. I wish I had better news. The guide links are there to help you to do what you can to support him to your best ability while and for as long as he is still going and the last guide is there for the case that he loses the battle so you can support him as much as possible and know what you can and need to do at every turn.
But the fact that your boy is slap-bang on the average life expectancy is testament to your good care.
PS: I am moving this thread to our monitored Health/Illness section for better ongoing support.
