Take pictures, and tell him how big he is getting. Make the 1st day a celebration. But you will cry, so try not to let him see you do it. My daughter is 6 now, and in full-day kindergarten. I remember her first day of preschool. I'm not the keepsake mom you are, but rather the one with the giant mental scrapbook who savors every memory. She was really shy and it was hard for her to leave me, but we managed. We went by the school a lot, did a class visit before she started, and developed a goodbye routine so it was easy to put in place when school started. There was no lingering, but just a kiss, a hug, and a high five. We still do it now! Also, when she needed just a bit more from me (they weren't allowed to have toys or special items in school), I wrote on her hand...just with a pen, I put a smiley face because I'm proud of her, a heart because I love her, and a star that I would kiss before leaving her and kiss as soon as I picked her up. All this was discreetly on the palm of her hand. When I picked her up the first day, she stuck her hand up to wave to me, and I got a picture of it - with all that I drew still in tact. I could almost cry now just thinking about it. But it's a happy memory. The time goes by quickly. Develop something special and you'll never regret it. My daughter had a rough week recently in kindergarten and was feeling emotional. I wrote in her hand and she told me she remembered when I used to do that almost every day. That's special to me. As far as preparation goes, let him be involved in the process. We did a school visit, like I mentioned, in the middle of a regular class so she could see what it was like. We drove by or went in the building frequently. We did the open house with the teachers and brought her along to be familiar with the classroom. She also needed a backpack, which we let her pick out. She also came along to shop for school supplies and was able to pick out some new clothes. We also did "Fun Fridays" and still do it now. To celebrate the coming of the weekend and end of a school week, we do something special on Friday - like have dinner out, or visit the library, or go to the park, or have dessert, or whatever comes to mind that seems fun at the time!
Make the most of your moments because they do pass quickly and you know you can't get them back. Don't be ashamed for being sentimental, but don't spend so much time keeping or scrapping that you get carried away and lose time you could be spending together now! Good luck!