If you want to eliminate unnecessary movement from a pitcher's delivery, then what was suggested for your son might be appropriate. Many young pitchers start off standing up straight as an arrow but as soon as they lift their knee and start their stride, they bend their knees and waist resulting in their whole body dropping and their head and shoulders moving forward (toward 3B for a RHP). What's actually happening is they are adjusting into a more athletic posture - one which they have the strength to maintain through their delivery while lifting the knee and moving toward the plate. If such a pitcher starts in this more athletic posture, then he doesn't need to adjust into it during the delivery.
So, if your son can't lift his knee and start forward without bending the knees and waist during the stride, then starting off that way may very well be a good thing for him.