You can buy lots of nifty things for hiding cash (passport sized money belt, actual belt with hidden zip etc.). Spreading it out (including small amount hidden in main luggage) as mentioned above is a good idea. I used to have pockets sewn on the inside of my clothes, but then I was a bit more paranoid and riskier countries!
You can just get a him an additional card on one of your credit cards for emergencies. That's really easy. Although you'll need to trust him a bit for that.
Scanning all major documents and and then e-mailing them to himself is a good idea too. Plus a few photocopies of his passport never hurts.
If he (rather than you) is apprehensive then one option is to book a short tour or something (e.g. learn to dive is a good option) that will meet him at the airport and help him find his feet in the first few days. Airport arrival is always the hardest bit - touts, taxi drivers, hot weather and a brain that just wants to sleep.
I used to take the Fox Mulder approach of "trust no one", but also doing everything I wanted to, so I that didn't regret it later.
Keeping in touch - everywhere will have wifi now, so a vaguely smart phone should help him keep in touch. Worth emphasising that whilst you'd like to know all the fun stuff he is doing, a short message saying "I'm alive" will suffice. Point being he shouldn't put off the latter because of the former. Equally, if he drops off grid for a few days, you have to try not to worry - there will still be lots of places without wifi, plus he'll be having fun!
As a parent, it is your job to worry until your dying day - my mother has suffered! He'll have a great time. And remember, even in the worst case scenario (which won't happen), you can always Weston Union money in a few hours.
Really can't help with the nut allergy.
Thai healthcare is relatively good, so just make sure he is ensured.
Finally, travelling alone can be a bit of a shock for some people. Everyone always says that you meet lots of people all of the time which is true, but he should be prepared for being along some of the time - to some this is the cost of complete freedom. Pack a book.