J. - Really, don't worry about whether your son makes it down the aisle or not. He will be adorable, and it will be a great memory (and hopefully get captured on video) no matter what he does. The great thing about a kid that young and cute in a wedding party is that nothing they might do can be wrong. I have 2 examples for you:
(1) My son was the ringbearer in my aunt's wedding 2 years ago when he was 14 months old. He wore a tiny tux with tails that was just adorable. My daughter, who was almost 3, extremely verbal, and very good at following directions (or so we thought), was the flower girl. She was to carry a flower basket in one hand and hold my son's hand with the other and help lead him down the aisle while he held the ring pillow (with fake rings) in his other hand. For that part of the wedding only, I was crouched at the end of the aisle in the front of the church to encourage them to walk to me. Believe it or not, they did perfectly at the rehearsal, so I thought it would be the same at the wedding. I didn't realize how having people in the audience would change everything. At the wedding, our nanny, who was sending them off up the aisle, unexpectedly ended up having to walk up the aisle herself like a streetsweeper to keep my daughter moving along and to pick up the ringbearer's pillow since, once my daughter started up the aisle, she immediately dropped my son's hand to stop and wave and say hi to anyone she recognized in each church pew she came to (and there were a lot of pews to walk past - and she stopped at each and every one!) ... while my son, on the other hand, kept right on going, but immediately dropped his ringbearer's pillow on the ground so that he could reach both of his arms toward me as he ran up the aisle with his arms outstretched and a huge smile on his face, repeatedly saying "Mommy, mommy!" Honestly, though, I think that the kids deviating from the script and being - well, kids! - was the highlight of the wedding, and so did my aunt and all of the guests who commented to me how much they loved what happened. I'll also bet that, as time goes by, none of the guests will remember much about the wedding except that part!
(2) Two weeks ago I attended the most perfectly-planned wedding that I've ever seen, thanks in large part to a perfectionistic bride (my friend) and the wedding coordinator she hired. If ever I would have expected the flower girl and ringbearer to do things correctly, it would have been this wedding. She actually had 2 ringbearers - 2 and 3 years old. The older one did his job perfectly, but the 2 year old stopped a few steps up the aisle and started crying when he saw all the people, while the flower girl and older ringbearer went ahead up the aisle and left him behind. His dad, who was the best man, had to step off the church platform/stage and walk down the aisle to escort his crying son up the aisle. Except for his heartbreaking little cry, it was the cutest thing and, honestly, stole the attention away from the flowergirl and older ringbearer who did their jobs correctly.
So don't stress about your son's "performance" at the wedding. Whatever he does will be perfect. Just let him do whatever he is inclined to do, and be sure to have your camera ready. It will be the most natural and cutest part of the wedding that everyone will remember fondly, and you will smile about it for many years to come. I'm sorry that I can't give you any advice about the tux issue though. Fortunately, my son was too young to have any opinion about wearing one or not. Maybe it would help if, the day of the wedding, he can see that he is dressed just like some of the other men in the wedding party since it might make him feel very grown up.
Good luck, and please be sure to let us know how it goes!