Why did the aliens return Barry so soon, in 'Close Encounters of the Third Kind'?

To me, this makes perfect sense. Given that we never know where the Aliens come from, how long the trip takes, or what happens to the people on the ship, I assume that the return of missing vehicles and such corresponds with the overall plot of the film. The aliens are returning stuff in an attempt to progress contact with humans. It is all stuff they took in an inquisitive manner (or maybe just to show they could). The reason Barry is returned so soon is that his abduction corresponds close in time with that final interaction. If nothing came of all that interaction, I would assume child Barry would show up years later during the next attempted contact. But this is all just speculation and opinion. Realistically, it is probably just an emotional tie that Spielberg wanted.

The simple answer is that Barry was taken very recently. When the aliens finally decide that we're worthy of official first contact, as a show of good faith they returned all of their subjects in the same state they abducted acquired them.

There's no specific information about how long Barry was gone, but it wasn't very long, albeit sufficient time has passed for his mother to have had a mental breakdown and lose considerable amounts of weight:

She did not look at all well. She had lost a lot of weight since Barry had been taken. More than that, though, Jillian had the look of someone who had suffered the greatest loss imaginable and was paying for it.

On his return, he's obviously the same age as they day he was taken:

One of the civilians, staggered by it all, said to the team leader, “They haven’t even aged! Einstein was right.”
“Einstein was probably one of them.”

_{^{Quotes from 'Close Encounter of the Third Kind - A Novel by Stephen Spielberg'}}