Arguing and getting a court involved every time there is a problem with access is going to do nothing but line the lawyers pockets.
You and your ex need to get together and arrange a sencible plan , the most important thing here is the well being of the child, it is they who suffer when parents can not agree on things like this.
I know what I am saying is a lot easier to say than to do but you really must make an effort for the sake of your son.
You can talk to as many solicitors a you like and get all the court orders in the world but in the end if you and the mother can not come to a mutually agreeable arrangement it will all be in vain.
Looking at this from the outside it is not all unreasonable that the Mum goes home to her parents for Christmas, you talk as if she has done it deliberately to spite you . A sencible arrangement would have been to agree that you have him the next day when there was public transport.
Again , I know it is hard but you and the Mum MUST both work at doing what is the best for the child , the interests of the child come before either of your own problems.
Remember no court can force the Mum to do something she does not want to do , no matter how many orders you get.
You say you could not afford a taxi yet you talk of getting another court order ? A court order will cost £2000 to £5000 and remember there is no legal aid for family cases available now . Even a solicitors letter costs £50 to £200 !
I am talking from experience my daughter went through a divorce with fights over access , it was not untill the solicitor told us that if we wanted to take the case to court we had to come up with £10,000 deposit that the two of them finally realised they had to start talking.