First of all, I tip my hat to you for your willingness and commitment to get trained and prepare yourself to be a better servant leader for youth. Congrats on completing two goals toward your ticket - keep going.
Now, as far as Jamboree selection let me try and give you a little more insight as to why tenure often beats training. Recruiting is the main focus of the SM and the ASMs leading up to the Jamboree. Tenure means more contacts to draw from, and more credibility with parents making that decision to send their son for a couple of weeks for a couple thousand dollars with virtual strangers. Also, if two men were from the same home troop, it reduces their recruiting pool to fill the 36 spots on the Jamboree roster.
Leading a group to the Jamboree is night and day different compared to going to camp for a week. Having more than a year's worth of over-nighters and week-long camps helps the council leadership feel confident in the SM's and ASMs' ability to safely lead the tour, and be able to take all considerations into account (such as kids with diabetes)to reduce liability and ensure a successful experience for all.
I would encourage you to let your son go, and to try to staff. There are lots of positions that need to be filled. For someone that is hungry for training like you are, become an NRA Instructor (and merit badge counselor), or RSO, and go help with shooting sports. Last I heard they needed over 1000 instructors and RSOs to help with shooting.
Keep your chin up. There will be many more Jamborees, and starting this year they allow Venturers, so you may still have a chance to attend with your son as part of the adult leadership for his group. If he attends this Jamboree, it also will help him be better prepared to be a youth leader for the next one.