he might have left of his own accord… six siblings is a lot, and if he’s fully feathered he might be right at the stage where they “fly” (read:fall) out of the nest and get fed by the parents for a few days. in the wild they usually hop around from branch to branch until they master flying.
is it definitely a swallow fledgling? they tend to have really good parents, that are semi forgiving to having their children moved too… we used to have to deal with them every spring and the ones that were displaced in stalls, we’d usually put in a big bucket in the aisle (and away from horse hooves) for the parents to feed. eventually, they leave - doesn’t take longer than a few days.
if you can reach the nest, you could in theory put the fledgling back if you think he is too young. birds don’t have the same sense of smell that mammals do and generally, accept their offspring being put back where they belong… only thing to watch out for is upsetting the rest of the nest, you don’t want to have to deal with stuffing 6 of them back in a nest…
a good time to do it is generally right after the parents come in, deposit food, and then fly back outside. you can usually slip them in right before the parents come back that way.