Can´t I buy the life jacket for my son a little bit bigger, then he can grow into it ? (instead of Mini- Junior, or Junior- Survival for adults)

We would strongly advise against doing so. This has to do with the circumference of a child's neck. If a child wears an adult's life jacket, the child will slip down inside the buoyancy chamber. The child's head will not be far enough above the surface of the water.