Oh Beau that must be so hard for you to hear :(
Of course the grass must seem greener to him. But in all these years I bet he knows it is you he can turn too when he really needs someone.
His dad probably gives him more freedom too I should imagine (like not being at home when he gets back from school - which may seem great until you end up having to get your own dinner every night).
Is there something specific that has brought this about? It must be difficult for your ds to voice this opinion (assuming not said after an argument) knowing it will upset you :(
How often does he see his dad right now? And is it fairly close to you? Why not start by having him go there a bit more often so your ex gets to see what the school responsibilities etc are and your ds can see a bit more of the reality of living with his dad.
So if he only sees his dad at weekends at the moment then perhaps you should try doing 4 nights at you and 3 at dads, the swapping the other week. So it's more of a shared care thing. Or a whole week and weekend each and then swapping. Assuming you are within a reasonable distance of each other/school.
And what will happen about school? This is important re his GCSE's etc. You don't want to be swapping and changing too much these next couple of years.
Perhaps agree to it for a trial period of a few months (make sure this includes say the easter hols and summer so your ex has to arrange childcare for his half and don't be helping him out with it, he has to take on thw whole responsibility if he wants it - thoguh obviously be happy to take your half of the holidays!).
Even if you have half the nights each you will keep the Child Benefit for now until a permanent situation is agreed.
Now is the time to get some training or look at your job situation if you're already working.
Now that your ds is 14 he can tell a court what he wants, should it get that far, and they will listen. It won't happen overnights, even with court, hence why I suggest you agree a trial period of 50/50 care moving to ds living more with your ex should he decide he still wants that.
Big hugs to you