Changes his action should be avoided [unless its "mixed"]
Run up with youngsters should be reassessed in pre season [as a minimum] EVERY season.
The reason is simple / obvious, as they grow bigger and stronger, so their stride will lengthen - usual sign is increase in f/f no-balls.
I use the Dennis Lillee method to establish - needs to be 1-2-1 or 2, 3 max.
Get them to run up and bowl, starting on the popping crease, running away from the wicket [must be fully warmed up first so that running stride has reached optimum - ground must be flat too].
Ask them to do this with eyes closed and no ball in hand, just using mental visualisation of delivery.
Coach to drop a golf-tee down where their f/f lands.
Player keeps doing this until they've achieved consistency to get 6 tees within the spread of your hand [about six inches]
Translate that marker to align to a straight approach to an over the wicket delivery point runnning back the other way [normal direction].
Next get them running in to bowl [eyes open, ball optional] from this marker observing f/f / popping crease position - anywhere between toe and foot arch breaking the crease is fine but I wouldn't want it to be the back half of the foot [to close for growth].
Again repeat until consistency is apparent and rhythm is present
Introduce a ball and place a cone on the ideal pitch-point
Check for speed and accuracy
Check for follow through
Usually takes 45 mins per players but I only do this with the frontline seamers - 4 or 5 max
Repeat next pre-season or with new players or if persistent f/f no-ball arises
Last and most important piece - from the bowling crease, get them to mark out distance using normal steps so that they can consistently put down their bowling marker in the correct / same place. For bowlers with a long run up this is harder - I carry a builders 30 metre tape measure in my coaching bag and we can measure it out before play and pop a golf tee in the ground - helps with confidence and consistency.
The key is that the bowler MUST feel comfortable in their run up - this will take out all the issues arriving in to the delivery stride, freeing their mind to concentrate on the tactics of each delivery.
Good luck ...
I'm off to coach in Uganda in April '13 but must first raise £750 for the Cricket Without Boundries
charity - your donation here
would be very much appreciated.