So far the responses have all been in regards to teaching your son basic strength training exercises. Although I think that is much better than providing 'contemporary fitness-based' stimulus (you know like fitness machines or 'aerobic'classes) my question is why are we centering on things like push-ups, squats and lunges?
While I have no contention with kids performing strength exercises, their ability to do so safely is grounded in motor skill aptitude - basically, if they can move well and demonstrate appropriate technique, limited forms of this kind of training is typically fine (I say typically because volume, intensity and even minor technical deviations could become very problematic for a 5 year old).
I think a more practical way of approaching this would be to develop/create some fun movement-based games (such as an obstacle course) through which your son can gain motor skill enhancement. You can also add elements of these strength exercises into the circuit or course - not necessarily in the form of '3 sets of 5 reps', but more like movement ranges within the course itself. Remember, the key in working with kids pre-puberty is MOTOR SKILL DEVELOPMENT!
Also be very careful with basic recommendations -
"BTW at 5 dips and chin ups are impossible but it may be beneficial for a real young person to do negatives for those two exercises"... no offense intended at all 'brian b', but there is likely a much better method for this progression. Instead of dips and chins (or the eccentric versions of them) with young kids, we need to regress the exercise down to an easier level. I ALWAYS advocate that children first learning strength exercises starting with VERTICIAL forms of pushing exercises and HORIZONTAL forms of pulling exercises.
Wall push-ups are an example of vertical pushes. They become especially important if, during a regular push-up, a youngster incurs an increased lordodic curve. Remember, strength is SYSTEMIC (it involves the whole body) - if the torso isn't strong enough to maintain a healthy and safe spinal range, then move to vertical pushes instead (and ALL of the progressions that go along with them). Same with chin-ups... move to horizontal variations with younger kids. Set a squat bar or Smith Machine at a low angle and have the youngster perform a rep or two of a horizontal pull (by lying on his back underneath the bar). This exercise also has MANY variations to make it easier or harder.
- Brian
www.DevelopingAthletics.com