It's not just that your son is entering a new (and more independent) phase of his life, it's that you are entering a new phase, too. Some sadness is to be expected. Not only won't he be around, not only will he need you less, it'll change your relationship forever.
The good thing is that orientation only lasts a few days, so you got a brief taste of what it's like, but then you get him back for a little while longer. You'll be that much better able to savor the experience - and your relationship with him.