How to win a child's trust back?

My ten-year-old cousin Ryan was abused in the past by his father who is, thankfully, out of the picture now. He's been out of the picture since Ryan was five. He suffered some pretty serious injuries....he still lives with his mom. Ryan is extremely ticklish (especially his feet) and last time I interacted with him I pinned him down and tickled him until he cried. I regret it now, and would never do a thing like that again because it might've been traumatic for him. Was it? Also....I'd like to know what SHOULD I do when I interact with him? ANy ideas? Is roughhousing okay, or playful tickling?