It's a solid time. So much can happen over a few years. People mature at different rates. There are always the kids who dominate middle school with a six o'clock shadow. They put up some big numbers during those years but usually plateau by the time they hit varsity age unless they were national caliber to start withs. The person who won our 8th grade conference meet in 55 seconds ran 52 in 12th. He already looked like a varsity athlete in 8th.
Proper pacing, training, and mental makeup will impact your son's 400m time. Some sprinters can't break 60 in an open 400 in 9th grade even though their open 200 suggests they can. They either power through the whole race or don't want to commit to the pain it takes to run a solid last 150m. The quality of training at the middle school level is all over the place.
A 66 in practice is still commendable. Who knows, maybe your son's teammates dogged the 400 while he dogged the workout prior. Maybe he could have run even faster for other reasons. So many variables here. The known quantity is your son enjoyed it and you can support that. Don't listen to the naysayers. Best of luck with the rest of the season.