That is definitely workable and how I would do it if I didn’t have a dtv but it looks like the 60 day will be reduced to 30 days so you may only get 5 months this way by the time the first 90 days is done it could be changed. Who knows 🤷♂️
I wonder where the mother is, as there is a court order in place it makes me think she is in the uk, you should get advice from a solicitor as I doubt you would be able to take the kid out the country without the mothers permission whether he is duel citizen or not
they normally are classed as your hold luggage with nothing more to pay. And definitely don’t need to pay import tax if you are on a golfing holiday. Just check the dimensions and weight. As long as you are within both you have nothing to worry about