I would also try and ask him things when he's NOT in trouble, and really praise him for telling the truth. A lot of times at this age kids "lies" are more of the way they wish it was, rather than an actual trying to decieve someone
You could even do a little truth/not true game at circle, so he knows exactly what he's doing. You state a few "facts" and the kids tell you if it is true or not--simple things like:
"S has on a blue shirt today"
"True"
It's raining outside"
"Not true!"
then talk about the difference in what was true and what was not true, and how we always need to be truthful so that people can trust us