There's no way, easy or hard, to honestly answer that particular question, without dropping one hell of a bombshell upon his son. He might as well just answer it honestly and deal with the fallout afterward since there was no way he was going to lie to his boy. Even if he could have come up with a believable one.

"I was there to rescue my grandson."

Which he failed rather spectacularly at, twice even. He could only hope that Henry's parents had more luck at success than he had.