All three of my kids are now grown, but all were in the gifted program at our schools. It was a FANTASTIC opportunity for them and they just LOVED it. It was more work, but also more learning. They all thrived in that environment. They did not have to go to a completely different school, but it was part of the time away from their regular classes. It was wonderful. I highly recommend it if your child is nominated. (And, I really think if your child is nominated, that probably means they are planning on sending him...they wouldn't be wanting to get your hopes up for nothing.) My oldest started the gifted program in the 5th grade, DD#2 started in 4th grade, and DS started in 1st grade.
I realize since our situation was different (not having to change schools) and long ago, you may not be inclined to put much stock in my 2 cents, but I do want to speak just a bit about the other things you say. You feel your son does not do well with change, but you KNOW change is a part of life, and eventually, he will be going to different school as he gets older and I am a little concerned that you may be "feeding" this attitude a little bit. Obviously your child is very capable (intellectually) and if the teacher did not feel he was up to the challanges inherent in the program (including changing school) the teacher would not have recommended it.
Another thing that sort of speaks to me is your being sort of upset that your husband told your son about this..it is YOUR SON's LIFE. He should be in on discussions about it. and yes, his feelings should be considered, but I think if you were to turn down this opportunity because he is timid about change, you and he will forever regret the missed opportunity. Privately, you could talk to the teacher about possibilities-- what if he ABSOLUTELY hates it? can he go back to his regular school? but I don't think you should make that possibility or that discussion known to your son. I think you should talk to him and encourage him to do it. I think this is probably your oldest child, and I can see that you are still in the mode of making decisions for your child and doing things that set the course of his life for him. This is perfectly natural...this is part of parenting. But, by the time a child is 10 years old, they are nearing the age of accountability, they are understanding concepts that have long and far-reaching possibilities..they need to learn to make decisions for themselves, especially decisions that involve themselves. (for instance, my MIL bought all of my husband's clothes for him, even when he went away to college....this never let him express his personality in the way he dressed or to experience making a bad clothing choice, or saving and working to save his own money to buy something he really wanted --and possibly learning great truths about how clothes aren't really worth all that, etc.)
I think the thing the gifted program did the most FOR ME, as the mother, was to teach me to HELP my kids get to the point of making decisions for themselves that would affect themselves. (I am not talking about chores or anything like that. Of course our kids could not dictate where we lived, where we went to church, what kind of food we ate, what the set standards of behavior or rules of the house were, etc.) But oftentimes, I would try to get them to THINK about possibilities. like--we would be at a store and in the car to go home.... should we go this way or that way? You decide. Can you direct me how to drive to get home? Would the back way really be shorter? Would the regular way be really busy this time of day? another example, my daughter wanted to make a quilt..I found myself influencing her choice of fabrics...when she said "whatever you want, Mommy" that's when I realized it would only be HER quilt if SHE made the choices to suit herself... They would not have been MY choices...but it did turn out beautifully and it is DEFINITELY her quilt!
one other example and then I'll end this long note. When we moved to our current house, my son had the opportunity to change middle school for his last year of middle school. we live right on the line between 2 middle schools in our district. He decided to stay at his old middle school and the MAIN REASON was the gifted teacher at the old school... he wanted to have that last year of her influence...he didn't want to change to the new school. In retrospect, he thinks it would have probably been good for him to have been at the new school and meet those kids before they all came together at the high school, but at the time, he wanted that gifted teacher and says he would still have made that same decision because of that factor alone. I just want to say, even though your child thinks he does not like change, and enjoys everything staying the same...he will HAVE to change at various points in his life and you can help him have a good attitude about change..you can help influence that...and this may well be the very best thing for him, even if he does not think he would like to change.
It would probably be really good to talk to other parents and kids who are in the very same program your son is being considered for...see what they think and how it is working for them and what benefits they see from your school district's program...
Good luck!