How about this ? You book in to separate hotels, your son knows you are there and can contact you if he is need of assistance (probably money LOL !!) but as has been said, Patong is nothing like Bangkok and is far more relaxed. This doesn't mean it is 100% safe, but where is ??
Think back to when you were 19 and it was a completely different society than today's. Try to think about it from your son's point of view as much as you can. How would he feel if he saw a girl he really liked, but was sitting with you in a bar ? Granted you might encourage him to start talking, relax and enjoy the moment, but what happens when he wants to partake of some bedroom olympics ?? The chances are in the hotel, you will have rooms very close to each other, does junior want to be bonking away with Dad in the next room ? Hence the suggestion for maybe choosing different hotels. Also, when you are out and about together, won't he feel like you're "cramping his style" ?? Sure there will be times you want to spend together, but out on the crawl, a young fella prefers to be with his own age group. Maybe you can take him along to the MTB and he can meet a few fellow board members that are there at the time who are a similar age. That would also apply to you as well.
It doesn't necessarily have to be 2 separate holidays, but at the same time you don't have to spend all your time together. Try to find out if any other board members will be there at the same time and see if you can link up with them. Also, it is very easy to make friends and start up conversations with other fellas there. This side of things should be easy to do and should hopefully mean you are both relaxed with the situation. Then your only real concerns are making sure he doesn't fall in to any of the traps that lads do when they are not yet worldly-wise like us old farts !! LOL I think, as long as he's got a sensible head on him, he will be fine and you are the best person to know if this is the case or not. If on the other hand he's got a bit of an attitude problem and has been in a few scrapes, then you need to keep the leash held a bit tighter.
I am not trying to imply anything by those last few lines, as I have no idea what sort of lad you've got, I am just trying to give some ideas and hopefully am making sense. You're the best judge of course and "holder of the golden wallet" but I feel that you will find that you are perhaps being a little too wary because of what happened in Bangkok. If you were going to Pattaya, I would say keep a very close eye on junior, but unless Patong has changed a hell of a lot since October 2004, then you will be fine and I hope you both have a great time.
Cheers
Diz