Bangkok is one of my favourite places, might be going there again next month 
Best shopping malls are MBK, Siam Paragon and Central World.. also there’s a great weekend market, Chatachuk Market, but he might find it unbearable walking around in the intense heat.. clothes and shoes are ridiculously cheap in the markets and at MBK and the people are lovely..
Check this out for loads of info: http://www.bangkok.com/shopping.htm