Hi OP,
I don't know if you are aware of this, but arlington's rec league has different divisions for each age group. If your son is on the more competitive side, you should try to join a team that is in a high division when he finishes ADP. The divisions are posted here:
https://arlingtonsoccer.demosphere-secure.com/programs/recreation/schedules-scores
By the time players finish up ADP, all of the ADP players will filter out to Arlington's internal rec league, so the league will be more competitive anyway because the same players from ADP will be in it.
If you're looking for a less intense travel program than arlington eventually, check out PAC. Premier Athletics Club. It is based in Falls church with practices at Luther Jackson middle school, falls church high school, and maybe some other nearby locations.
Middle school soccer is extremely competitive in Arlington. The middle schools have several hundred students in each grade, and many, many players who play on Arlington's travel teams all want to try out and be on the team. Making the middle school roster is a big accomplishment just by the sheer number of players who try out.
It is not a place to learn soccer, it is just 2 months of the year, the coaches are more like PE teachers, and some of them coach for Arlington Soccer in some capacity or another.
So, I would not worry about Arlington middle school soccer at all. When ADP ends, just try to find a higher division team to join, and if you feel like taking it a step further, check out PAC.
Also, by the high school age groups, all of the less serious, less motivated players have moved on. The high school divisions of Arlington's internal league are very competitive, especially division 1, which has former travel players who dropped out, and talented players who should be playing travel but never did. The high school age group teams offer plenty of competition.