I would like to add some comments to those made by KFdancer.
The starting point, for UK tax purposes, is to consider residence. This is determined under the Statutory Residence Test (SRT), and includes the split year aspects. The SRT is complex, but its application can be straightforward, depending on individual circumstances.
If he is resident, then the worldwide income is taxable in the UK.
If he is not resident, then the UK earnings are taxable, although this taxing right can be overridden by the double tax treaty.
The best approach is usually to set out dates of presence and work in the UK, to allow the analysis to start.