Yes I have gone through this exact thing. I know what is wrong with your boy. It is "Hemangiosarcoma". My own dog developed that.
Yes, it is a terminal illness. It is a cancer of the epithelial cells -the cells that line blood vessels. It centers on the Spleen, as that is where there is a high concentration of blood circulating through.
But it can also be on the liver, and elsewhere in the body. My girl had Spleen and liver involvement, and three subcutaneous ones.
It rarely gives any symptoms until the first internal bleed happens. That's when a hitherto healthy fit dog will suddenly collapse with white gums. That is usually the first sign they have the disease.
Now the weird thing about it is that although a dog with that is terminally ill, they can be fine in between bleeding "episodes"!
Sometimes the first bleed is the last -as that can be a massive bleed, and they are on their way out. But most often they are smaller "slower" internal bleeds, the dog recovers after a couple of days, and goes back to normal!
However the internal bleeds will recur. How often? It's impossible to say. Sometimes days later, sometimes weeks.
When a bleed happens it can often (though not always) be managed by total rest, allowing the body to re-absorb the blood in the abdominal cavity, which does happen naturally. Sometimes there is a slight anemia afterwards for a few days, and there can be a touch of jaundice, but once the bleed stops, the dog starts to recover and enjoy life again.
That is exactly what happened with my dog. And she had a short period after that first bleed where she was still very much enjoying life.
Although I knew she was on "Hospice care at home", and daren't for one moment, take my eyes off her. I knew it was like living with something that could go off again at any time. But she enjoyed her last days very much, enjoyed her (gentler) walks, slept and ate well. Had lots of steak and nice food!
It's hard to advise you what to do. Not every case will follow the same pattern. Some dogs can live on for MONTHS -and I have heard of a few rare cases where they lived for over a year and more, in great quality of life generally. But yes, their life was interspersed with recurring internal bleeds now and again, which resolved with total rest until it passed.
You know your dog. You know how far he wants to go, when he is down and feeling rotten, and you also know how he is when there are no bleeds happening. If that quality of life declines too far, or the bleeds start to come back to back, then you will have to have a VERY FAST re-think.
One way or another you and he will end up with a very serious bleed one day. You will have to be so watchful, and able to communicate -by phone -with your vet, or a night duty vet, when that happens. You may need to take action at a moment's notice, and of course that will be very sad and you must be brave.
If you are a person who needs to work full time, and so he will be left alone for hours a day -then also have a re-think. Because he could start with a bleed while you are not at home.
I do advise you to talk to your vet about the options, and what to expect, and ask if there will be support/advice at a moment's notice, even in the night. Because you may one day need it.
Forget chemotherapy. It doesn't cure it. Removal of the Spleen won't cure it either. The cancer travels through the blood vessels and has already spread as far as it can go. It is no longer just in the Spleen.
I have heard that a herbal remedy called "Yunnan Baiyou" (not sure if I spelled that right!) can help the bleeding a LOT. It might be worth trying it? Research that. Google it and see what you feel.
Kind thoughts. I understand. Bless your dog! I love Border Collies.