I'm so sorry that you lost Ollie so suddenly and so young.
Rabbits are such fragile creatures, particularly when so young. By the time they are showing signs of illness, it's usually quite advanced. Diarrhoea is very serious in rabbits, so you were absolutely right to get him straight to a vet.
If you are thinking of getting more rabbits, I would get advice from your vet about any potential issues of disease carryover so that it doesn't happen again. Sometimes it is wise to leave a time gap, and have a thorough disinfect before bringing in a new rabbit, particularly if you suspect something like coccidiosis
I would also look at a different source so that you don't risk the same infection being brought back in from the same place where Ollie came from, and be very careful about sticking to exactly the same diet they are used to for a few weeks before you introduce anything different.
RIP Ollie