Does he seem bright and alert?
Is he drinking water? Any vomitting?
Is he still on any medication following the surgery?
What was the reason for the splenectomy?
Thanks for the extra information.
Haemorrhagic diarrhoea wouldn't normally be associated with a splenic disorder specifically, so I suspect this isn't necessarily associated with the surgery - perhaps more so a reaction to the antibiotics, anaesthetic, or even something completely unrelated.
If blood in the poo is bright red and fresh, this suggests an inflammation of the wall of the large intestine - such as the colitis that you mentioned. If he seems bright and alert and keeping himself hydrated you may be able to manage this at home initially and see how he goes, although without being able to examine him I would always say that you should contact your vet if you're unhappy with how he's doing.
The most important thing with diarrhoea is to keep hydration up, so encourage him to drink little and often. I wouldn't;t feed him for a couple of hours, and then offer something very bland such as some chicken and overcooked rice or pasta, and only in very tiny meals every hour or so - just a few spoonfuls at a time. This should give his guts just a small amount to deal with at one time. If he remains bright and happy, is not vomitting or losing large amount of blood then see how he goes on this and call your vet for advice when they open. However, if he deteriorates, then give your vet a call.
I hope this helps,