It would be useful to know what age group because the role of 6 changes depending on the number of players. My son (the GK) is currently having this issue with his sweeper (who is not at all small and actually one of the better players on the team) because it leaves him vulnerable to the one v one, and having just moved onto one v one techniques, he doesn't have a whole lot of confidence yet, but it's getting there.
Moreover, at the older spectrum of the littles, it's important for the 1 and the 6 to talk and have communication so they can protect the center channel. As others have written, it's difficult to teach them to play aggressively (let alone force them)...often times its just a bit their nature, though aggressiveness can build as confidence develops in the skill set. At the U9, for example, coaches can place the 6 there either for a more defensive posture (treating him as a true 6) or as an 8. Some coaches who are offensively minded (and are confident in the skills of the GK) will want a kid there that can pass the ball well (since the 6 has access to the full backs, the wingers, and the forward). Others (particularly those with weaker GK) might want someone there who is a pure defender and can tackle aggressively. Your coach probably sees something in him, particularly his passing ability, that they want in that slot....rather than have him switch, why not dialogue with the coach about why he has your DS in that slot and what he perceives as the strengths and weaknesses? If the coach is using the Calsouth system, the emphasis is less on individual training and so may be expecting your son to be aware of certain difficiencies and to be working on them in private.
If the coach has the 6 in primarily a build out role, and opposing teams are still scoring, then it's not just an issue with your 6. It's an issue with your 1 and your fullbacks too. The reduction in play time may be because the coach is switching from a more defensive than offensive capacity in that role, or maybe sees that your son isn't working on the weaknesses in his game outside of team training. The question you have to ask is are his weaknesses something that would affect his role in any position, or is it specific to his current position. A discussion with the coach, and a willingness on your part to accept some frank criticism, should give you some clarity.