We were there the week after Memorial day. My son also was desperate to build a light saber at Savi's. This was the first time that we had been since Galaxy's Edge opened, and I didn't consider that I'd need to make a reservation for it. So, naturally, when I realized that I needed to make a reservation for it, it was completely booked for the entirety of our trip. I went to Mouse Watcher and setup an alert for the dates we were planning to go about a week before the trip. Within a day or so I started getting alerts about single reservations opening up. Two days before we left, I was able to get one of the reservations. If you're son is really set on it, and you're having trouble getting the reservation otherwise, it is well worth the $5 it cost. I was actually quite surprised with how often it was sending me alerts about reservation availability. The only real caveat is that you have to be quick on the trigger. There were a couple of times that I got alerts and within a minute of the alert coming in, the reservation was gone. I'll also say that the frequency of the alerts increased markedly as the days approached. I guess more and more people were canceling at the last minute.
Good Luck!
