His creatures will not have summoning sickness and can attack normally.
The rule informally known as summoning sickness says that a creature permanent cannot attack or use abilities with the tap or untap symbol in their costs if its controller did not control it from the beginning of his last turn.
However, that rule does not apply here, since your opponent got his permanents back during your turn, even if it's at the end of your turn. He will start his turn with all permanents in play, so summoning sickness is not an issue for them.