US Citizens stuck in UK on a shared residence order

[drzeus]

My step child is a US born citizen who was born in San Diego and lived in the US until he was 5. He and his mother and father moved to the UK 5 yrs ago and then the parents separated, a custody issue arose in the UK and shared residence was ordered by the courts with mother primary carer. Now I have a baby daughter with mother who is a US citizen and we would like to return to the US, my stepson is adamant that he wants to return to the US as soon as possible but his biological father will not give permission (biological father is also a US citizen but dual nationality in the UK). We are seeking advice re the Hague Convention as the child is US citizen would the US be obliged to return the child to UK against his wishes if he entered the US and refused to return to the UK? Also we are seeking a possible 'leave to remove' order as mother and child both wish to return to US. Any advise on best steps to take would be greatly appreciated.