Can Mini's eat fresh grass?

I have a Miniature horse (Rumor) and we have a strip of grass at the end of our arena with a lot of fresh, good grass growing on it. Can I hand graze him or turn him loose on it for half an hour?
I have heard mixed stuff about if it is okay to turn mini's out on grass.
Can it cause them to founder?
I know Rumor would love it I just don't want to hurt the fat little guy!