While your vet is right, no one really knows how contagious FeLV is - it's shared exposure, whatever that means. Unfortunately, the more 'immature' a cat's immune system is, the easier it is for them to contract it. And it can take up to 90 days from exposure to it showing up in the blood stream, which is why they said three months. I'd be concerned, but one sniff is not likely to pass it - probably. The pet store is very irresponsible to have cats with FeLV in there where people bring their cats, and quite honestly, I'd be finding another place to get his nails trimmed. The fact that they are offering to have him tested doesn't make up for their irresponsibility, not in my mind.