He has the pardon power for as long as he is in office. He loses no power just because the House formally accuses him with articles of impeachment.
Article II, Section 2 provides:
[The President] ... shall have power to grant reprieves and pardons for offenses against the United States, except in cases of impeachment.
He gets the power back if is acquitted is the way I read it. If the house doesn’t send the articles to the senate, is he impeached? We’ll find out in a few..