If the kid is mentally engaged but simply is slow and non-aggressive, you can put him as the last defensive player before the goalie. You’re creating a new formation with a single "last stopper." I’ve had very good luck with this kind of kid by telling him (in a one-on-one conversation) he only has one job but it is the most important one on the field.

Then describe the job like this: It is your job to make sure the other team never shoots on our goal. No matter where the other players go, you stay at the top of the goalie box. When the other team comes down on a fast break, you always keep your body between the ball and the goal. You need to kick the ball out of bounds as quickly as you can — out of the sideline if you can, but even out of the end line if necessary. When you kick it out, kick it as hard as you can. Key rule is NEVER kick it into the middle of the field.

This essentially turns the kid into a second goalie. When he kicks the ball hard out of bounds, it buys time to let the rest of the athletic players get back into position and set up the defense. This strategy can turn a liability into a strength that can really keep the goals-against numbers down. And it makes the poor player feel he is doing something really important and he can succeed at it, which keeps him very happy and valued on the team.

Over time, it can be so effective that the offensive players develop confidence that they don’t always have to race back all the time and they can stay upfield like they should. This results in more offensive opportunities. I’ve managed some undefeated seasons with not a lot of talent in rec leagues doing this.