S.M.
It is probably better to have the 504 Plan accommodations in place at the beginning of middle school, due to the transition and the fact that he will have multiple teachers. The 504 Plan can be written listing accommodations that should be used AS NEEDED, so they'll only be implemented if your son is struggling. If you choose to not have a 504 Plan right now, your son will continue to be eligible for one at anytime as long as he continues to have a medical diagnosis (like ADHD.)
Good luck.