But they do have the right to decide this is best for their son,
No they don't.
If you decide, for example that it is best for your child to keep them in a broom cupboard, throw them down the stairs regularly or feed them nothing but bread and water, you'll soon find that your rights are significantly constrained by British law.
You need to accept that children are people too and they usually grow up to be adults. Morally, if not legally, you do not have the right to do things to them that are not in their own best interests. Children are not your toys to indulge your fantasies on.
and the law backs them up.
The law is wrong on this occasion.