So earlier this week, around Tuesday, I noticed Osiris acting a bit oddly. That, plus his recent weight loss worried me, and I phoned the vet up. They did a blood test, and then phoned me later on and said that Osiris had diabetes. So I've been giving Osiris insulin (after being shown how to give it) for the past few days (twice a day). He does show some signs of improvement I think. Also, I've switched them (meaning Osiris and Bastet) completely to wet food as that's better for them.
I'll be home testing hopefully by next weekend. Just need to get the meter and stuff. I hope things will get under control for Osiris as I worry about him.