5 is a very mature age, even though many kids aren't treated as mature by then, because they are still battling immature behavior patterns due to their parents. Not saying it's your fault, but there probably are changes that need to be made in his day to day since you are certain he doesn't have ADHD (and even if he did).
Firm consistent discipline should not be removed, but is it random? Do you only get firm with him when you're totally fed up rather than preventatively? Is it unpredictable and angry? Always remember, if you're mad, you're not teaching, and you shouldn't be disciplining.
Remember, at age 5, he should be acting well most of the time (hang in there with me), he should be having mature responsibilities, some tough chores, etc and LOTS of boisterous rough play. You should be praising him a lot and treating him like a strong independent young man, and then ONLY when he is outwardly defiant, etc, should you give him a firm consequence after one calm warning EVERY time. This would keep the discipline to a bare minimum if everything else is aligned within a healthy happy lifestyle with plenty of sleep and nutritious food, etc.
Stay in charge, and keep the firm discipline, but if you are constantly negative and angry and punishing, it will lose all effect. You need to be calm, confident, positive and loving MOST of the time, so he knows your consequences are logical and fair based on his actions. Always give him a clear expectation, and clear explanation of what will happen if he chooses the wrong action and then follow through firmly while staying calm. Always use the same, firmest thing, so the consistency sets in faster. He will catch on and get in the habit of making his own right choice. If his dad is around, MAKE HIM TAKE THE LEAD on discipline. it is imperative with boys.
It's hard to give advice not knowing what your discipline style is, but "tried everything" often means lots of varying chaos, which won't usually work on difficult kids. This site and book has GREAT firm, but fair, logical guidelines for your little gentleman:
www.backtobasicsdiscipline.com