He will be liable to tax in the UK as a first point of law as he will be paid from the UK - then when HMRC consider whether he can be awarded NT (No Tax) status - they will look at whether any of his work or duties take place in the UK - to which the answer is yes,
So although he could spnd less then 90 days in the UK per tax year -0 he will perform employment duties in the Uk and its this proportion of income that will remin haing a UK liability.
So I advsie
Alert HMRC to the fact you have engaged this employee in the normal way and operate normal PAYE against this income, then ask this employee to complete form P85 (which does lean towards employees whom have worked in the Uka nd are now leaving the UK to work abroad) but will allow the information HMRC need to establish a tax position, and if he thens ends this form with a covering letter stating that he has always lived and worked in the USA until this time and advsie them what the arrangement will be - so they can determine a UK tax position for him.
Its likely he will remin fully liable on the income throughout the year and then be awarded any non residency position against the income not earned whilst in the UK
But note that if he comes to the UK for holidays and exceeds the 90 day rule, then he will have a full UK liability on all the UK income
As far as the USA is concerned I am sure you can appreciate we are UK tax experts on the UK tax forum so he should alert the IRS of this situation and if they state he is liable in the USA the under the double taxation agreement the UK have with them they will take the UK tax suffered into account so he is only charged the once at the highest rate
Let me know if I Can assist further
Link here for form P85