I don't know a whole lot about FIV but I know a bit about FeLV. FeLV cannot survive long outside the body. The biggest risk of transmission of FeLV is through shared water/food, litterboxes and intergrooming. It is transmitted through the saliva, fecal, urine and body fluids. If your cats are over 1 year old it is highly probable they have built up an immunity to it. 60% of all cats over 1 year old are naturally immune to FeLV. Out of the 40% remaining that are not naturally immune 90% of those will take to the vaccine leaving 10% of the 40% who are not naturally immune that the vaccine does not work on.
The stats came from my vet in Canada if you are wondering.

As for FIV this is transmitted through blood and sexual contact I BELIEVE . So if he were to bite one of your cats it is possible that they could become infected. Jenn knows a ton more about FIV though as do a few others like Lizzie.

I would release him in the bathroom but be super careful about knowing where your cats are when you go in and out and always have your hands ready to grab one or the other. I think seeing it is only a couple of days he wouldn't want to venture out so soon. But I am sure you will get lots of advice and opinions.

Good Luck and you are doing a wonderful thing for Tigger.

Michelle