Jason Porter of Blevins Franks advises that the tax treaty does discuss less than, or more than, 6 months in the UK, and which country has the taxing rights. But, there is also a UK rule around spending more than three months a year in the UK on average, and it is likely he will fall foul of this, and therefore be classed as UK tax resident. There is obviously a huge difference between rates of tax in the UK and Brunei, and it is in his interest to get it right. Otherwise, he may make insufficient provisions for his UK liabilities (if his employer does not do it for him).
He will need to explain to his new employer what his plans are, re returning to the UK for his weeks off, and that he will be present in the UK for 3-6 months a year. They will then hopefully deduct the correct taxes and pay it over to the UK tax authorities on his behalf. If not, they should tell him what they will be doing, so he can plan to put aside the UK tax and NIC he will owe.