If he had a stomach virus and you were to contract it, it can take anywhere from 12-48 (and I've even heard up to 72 hours in some cases) before you'll feel ill.. Usually after 3 days of being exposed to someone who is sick, I tend to ease up a bit.
The good thing is, you're probably in the clear unless you touched something he had touched (door handle, tv remote, etc.) and then touched your face. SV is spread through the v* and d* and if someone is ill and doesn't wash their hands after, they can contaminate objects they come in contact with.
However if you were just in the room with him, you're probably okay
I completely understand your concern though, and hope that your nervousness and n* goes away!