You can never have too many name-tags/labels. There is a natural temptation to pack a lot of clothes and other things given the distance between school and home, but it might be better to not overload him. Add a couple of spares by all means since socks etc may go missing for a while before they eventually find their way back to him. Despite the laundry bags, things do go missing though typically not permanently. Don't have anything delicate or require dry cleaning only. A woolen jumper went down about three sizes by half-term! It was nicely put in one of the letters for new men: school laundry is "efficient but not subtle". It may be a good idea to have a few spares already name-tagged at home or at his guardian's so that you can send him replacements at short notice. Football gear often end up in the second hand shop almost new, unless of course he is very keen on football (different rules though).
If you do not have an address in the UK, getting a SIM card subscription may be tricky, but O2 has perhaps one of the best international top-up cards which has 1p per minute voice rate to most Asian countries. Reception in WinColl is not the best. If he is in College, he will have to go outside to certain spots to get a signal. O2 also has pretty good data packages. Phone usage is pretty strictly controlled. For the first few weeks, try not to text him during school hours just in case he forgets to silent his phone - you don't want to cause him to lose his phone if it rang say during chapel or meal times. It happens.
They are prone to misplacing belongings or have them misplaced by others (often with good intentions).
Airfares may well be cheaper if you buy London-Asia-London instead of the usual Asia-London-Asia return. If he is going home for half-term and Christmas, you should book your tickets now. Many airlines are also very fastidious about procedures for children traveling on their own. You may want to check with them. If he is a British or EU citizen, they can get very jittery about him not having a return ticket back to the UK (child abduction fears).
There are a few very reliable cab drivers in Winchester who are well organized and very used to taking overseas boys to and from the airport.
They discourage boys from having ATM cards. For boys living abroad, it is probably necessary to have debit and ATM cards. For Asia, HSBC is probably your best bet.
Swimming is the main sport during his first term. So, be sure that he has the gear for that. The pool is said to be quite heavily chlorinated.
If he is in College, a desk lamp is almost redundant.
Spare shoe laces may be a good idea.
There are probably a whole range of other things but these are the main ones that come to mind.