Am I correct in thinking that all the income of the trust will be assessed on him despite his residence status?
No; non-UK source income will not be assessable on the settlor. UK source income may be subject to income tax on the settlor depending upon the type.
Probably preferable to avoid UK source income to avoid UK tax problems.
Offshore bond feasible particularly if the 5% withdrawal facility is enough re school fees.
As the trust is non-resident, settlor is non-resident and bond offshore then trustees not liable to UK tax and neither is the settlor on any chargeable event.
My concern is whether any of anti-avoidance provisions apply as and when trust monies applied in providing kids with a benefit.