Colleen,
The one role I've never served in is as a coach. But the tournament is only 6-8 hours of the total FLL experience. I'd let the boy participate fully--your team will be stronger for it. You'll probably be surprised how well your team can adapt to filling in for a member who is available for all/part of the tournament day.
The judges don't count who isn't present--they only evaluate what the team presents. We're used to kids getting sick, having family emergencies, sometimes even moving cross-country!
Encourage your team to think creatively--"What if Johnny can't be there for our project presentation? How will you be able to still make it work?" See what they come up with. Five of the 30 judging minutes are scripted--the rest are generally free form Q&A. Check out the Core Values rubric (
http://www.firstlegoleague.org/sites/default/files/Official_Event_Info/CombinedRubrics.pdf) and the element of Inclusion. Better teams have members who can describe how a missing member contributed to the team, so it's not a big deal.
If your team will rotate members in and out at the table, then it's simple. Someone else can substitute in. Even if the substitute and the original partner don't fully mesh, it's okay--only the best of 3 scores counts anyway.
Finally, look at the FLL Core Values: "What we discover is more important than what we win." I volunteer for FLL because I really value seeing what the children can discover and how they can grow in their skills. Recognizing achievement through awards is important, but the awards ceremony is not where the learning, growing, and enthusiasm happen. Those happen during the season and in talking to the other teams at the tournament. The judged and scored events are a small piece of that.
Steve Scherr
VA/DC FLL Referee Advisor