I recognise that your child has cerebral palsy of the diplegic kind; meaning the legs are more affected than the arms. There is spasticity which is more in the hamstrings because he has a crouched gait. It is also possible that his calf muscles are tight too.
I would recommend that he be seen by a Paediatric Neurologist/Orthopaedic surgeon.
Botox is beneficial and the earlier that it is injected (2-4 yrs), the better.
What one has to see is that if the contractures or tightness is fixed (permanent, in lay term) or not. If not fixed then Botox injections can be administered with subsequent serial casting. The effect of Botox lasts 6-8 months, and in some patients it may have to be repeated. Barring the cost, Botox has no appreciable side effects.