Congratulations on the marriage of your daughter. Very exciting! And, in Italy no less. How romantic!
I left my 3yo for the first time when I went to London for 10 days. I was anxious just like you are and bought enough gifts to give him one each day. He stayed with my parents and they made the executive decision not to give him a gift each day. My son did so well and had so much fun that they felt he didn't need it. I was upset about it, but realized my son didn't need it and it was really my own personal issue. I have since left my kids several times with no issues. They love staying with their grandparents (and they don't do all the extra fun stuff I know others do when they have their grandchildren).
I have learned with my own children and also two little girls I watched for a week (all are between the ages of 3 and 6) that they become a little "mommy-sick" at around day 5. It's now at that time that I do a little extra to help ease them. For me, it's a special book to be read and I try to encourage my parents to plan something extra special around that time. Regardless, my kids do so much better than I do and I wouldn't worry too much.
I am also very up front with my kids. I show them on globes and maps where I'm going and we research on the internet a little about the location. They get excited about it and I always bring home a small souvenier. These souveniers end up at school to show their friends and they talk a lot about the place we visited. It's a great learning opportunity.
One other thing I learned with the 2 little girls I watched is that we should've done a practice run before the big trip. They had never been away from mommy and the first night was a little rough. The older girl (6yo) cried and couldn't get to sleep. We worked through it and everything was fun after that, but we realized we should've had a one night sleepover before the big trip.
Have fun with your adventure. And, don't worry. Regardless how your son reacts, he will be okay!