I understand your concerns. Boys are so wiggly and wild and unpredictable. My son, Mason, at the same age of your son was asked to be the ring bearer for my brother's wedding. It was a very small wedding with only 12 people present including the bride and groom but we did practice with him ahead of time.
We didn't have to wear formal attire so that was a plus but in your situation, I like the suggestion of showing him a picture and telling him it's what big boys will be wearing. I suggest having him sit with you during the service that way if he does get wiggly, and he most likely will, he won't distract from the bride and groom. You may have to take him out for a walk so be prepared and don't feel bad if you have to do this. Boys at even older ages, okay even men, just have difficulty sitting for long periods of time. It's just how they're wired.
When it came time in the wedding for Mason to walk to his uncle and hand him the ring box, he hopped down from his chair, took a step, and then pelted the box across the front row to his uncle rather than handing it to him. Even though we had practiced what he was suppose to do, in that moment, he did what he thought he needed to do. It was actually considered a humorous highlight during the ceremony and we talk about it still today.
Bottom line, the best advice is to certainly talk about it with your son and even physically walk through a mock ceremony. Make it a game so it's fun. That way he knows what's expected but has fun at the same time. And know that with all your preparation, it still may not go as planned but be flexible and love on him despite what happens. It's certainly a lot to ask of a little one.