Conditionally promoted sounds like the child may not have met the criteria for promotion. You need to ask the school counselor or administrator to explain this term and explore what interventions are available to assist you child.
This is definitely something that can only be answered by your son's school - not an advice board. You need to find out from the school what the stipulations of his promotion are and do your best to meet them if he is indeed ready to be promoted to the next grade level. If he is not ready to ascend perhaps there are still areas that should be addressed by you the parent in ensuring that he is meeting the expectations of his grade level and the school culture.