How do you loosen up your child?? My son is always stiffing up his legs and back for everything

You don’t. Eventually he’ll start to wanna loose up.

It took me a minute to understand what your were trying to say, but at 2 months old it is normal for the baby to be "stiff". It will eventually go away on its own through growth and development. But, in the meantime you can give him little baby massages with coconut oil after warm baths. There are light stretches you can do also. Speak to his ped and they can advise you. Best wishes.