Emma got these the last time we went to the park and went a little off the trail...at least I think they were grass burrs-some kind of burr anyway
I found the best way to get them out of her cotton coat was a wet washcloth, followed by a wide-tooth comb, and then a normal brushing. Leave-in conditioner might help too?
EDIT: just realized that you were asking how to get them out of the YARD and not out of his hair. Oh well! Hope this helps if he gets any more!
Just remember-soon enough we'll have snow

and there won't be any more burrs to catch on them...next we'll be worrying about the snow chunks that build up along their legs and tummies!