My DS goes back to his boarding school in three weeks, he's boarded for 10 years I'm already starting to feel sad. In a way I think the run up to him going back to school is the worst bit.
I think about all the advantages, the numerous things he gets that he could not have if he stayed at home, the friends he has, the marvelous dedicated staff, etc. I believe my DS is receiving a fantastic unique education, and that boarding is a positive life enhancing opportunity. This is what will carry me through the next three weeks and beyond.
You need to believe in you and your DS's choice, there will inevitably be the odd rough patch, children do become home sick, fall out with friends, get told of by teachers (as they do at all schools) and wonder whether they've done the wrong thing, if you're crying and anxious you'll make it worse. I'm believe in not hiding our emotions from our children as a general principle but if your crying about it you will worry him, he will go off feeling anxious about you and he doesn't need this especially in the begining when he's getting used to boarding. My DS know we miss him terribly but he doesn't have to worry about us, there's a difference.
Two bits of advise. Boarding is also exhausting particularly the first term for new boarders, it seems longer than the others, most kids (and staff) will pick up some kind of cold type bug, the evenings are getting shorter and the weather changing, it always feels a funny term, just when children are exhausted of course Xmas parties and all the excitement etc come along at the end of term making tired children and staff even more tired. Don't be overly worried if your DS by Xmas or even half term is quieter than normal. Secondly time just seems to go so fast. I clearly remember my DS's first night at his boarding prep, his 11+ pre test and sitting the entrance exam at 13+, I also remember his first day at his senior school which feels like yesterday but the rest is a rapidly moving blur. In three weeks time he start in yr 12, now we're counting down to the end, starting to seriously plan where he will go next. If your DS is full boarding in a few weeks he'll be home for an exeat, then it will be half term, Xmas and before you know it it will be July and you picking him up for for the wonderful long summer holidays and in what is begining th feel like little more than the blink of an eye you'll be picking him up for the last time. Enjoy his time at his school attend plays/concerts, even if he's not in them we go with our DS, watch him in matches, attend social functions the school organises, enjoy the school as much as he will, then you will both emerge 7 years later with positive memories.
Good luck.