a customs duty,the amount of which is a percentage of the value of the goods (so tell him to be sure to bring an invoice showing the full purchase price) and also VAT at the full UK rate of 17.5%.
I have visited Sabah a couple of times on business myself, and saw similar 'bargains' there but.......I have to say that I was far from convinced that the machines were all that they were said to be. Your son will obviously act on his own judgement, but I would strongly advise caution in this. If the machine (which will have cost a good deal more than £300 by the time it's cleared HM Customs & Excise) goes wrong at any time your son will have absolutely no recourse to either the retailer or the manufacturer - he'll be on his own.
I truly hope that your son has indeed found a real bargain,and he may well have done just that, but consider this - if these machines were really as good as they seem we would all be buying them, even after the import duty and VAT have been applied, but we're not....and there must be a good reason why we're not.