By taking those factors into account, the court might decide that the best thing for your son would be to permit him to start a new life with you abroad. However, you should also be aware that the courts consider the 'Principles and Practice in Regulations and Guidance'. Two of the provisions are as follows:
# Family links should be actively maintained through visits and other forms of contact. Both parents are important even if one of them no longer lives in the family home and fathers should not be overlooked or marginalised.
# Continuity of relationships is important, and attachments should be respected, sustained and developed.
Those provisions could be used to add weight to your partner's application, so you would be unwise to assume that his application would automatically be rejected. (If your partner applies for a Prohibited Steps Order, it's essential that you should seek professional legal advice straight away).
So, it's not possible to give a definitive answer to your question. Certainly he has the right to apply for a PSO but, until the application comes before a court, the outcome must be regarded as uncertain.
Sources:
This information has been based upon several web sources, with this being the principal source:
http://www.spig.clara.net/misc/ch-act.htm
Chris