I am sorry nut to put it bluntly the document that you have is worthless as you have no right to change the name of the Child without the agreement of the father or an order from the Court.
You have neither - all you have is a document prepared by a commercial firm who have charged you money for something they knew you woudl not be able to use.
On that basis you cannot change the name on the passport AND technically you should not have changed his name with the school
If you do wish to change the name of your child then you do need to contact your ex and ask if he will agree to the change.
IF he will then you can download a document form the net (for free) and get your ex to sign it as well.
Then you can get the passport.
If he will not agree then you can make an applictaion to the court - but this is unlikely to be granted at this stage.
You asked about him waiving Parental Responsibility.
The only way that this is posisble is if your husband adopts the child.
Again you will need to ask your ex if he will agree. Since this would mean he would never have to pay maintenance he may agree in
which case you can proceed with an adoption.
If he will not agree to that then you may wish to consider is having a Step Parental Responsibility Agreement signed by your ex (and you) to give your husband
Parental Responsibility for the child.
If he will not agree to that either then a Court would certainly grant it