If it can be sold why don't you sell it?

I don't think you would because once you make the change that he is the lead passenger then my understanding is that all communication would be with him only. Reading between the lines are you saying that you fear that if he becomes lead passenger then he won't sell the holiday on but change the name of the other passenger and go on this trip regardless and thus leaving you out of pocket? If that is your fear then ask him to pay you what you've already paid out up front before you make the change re making him lead passenger.

The only guaranteed way to remain in control is to refuse his request and as nickmacuk says, sell the holiday on yourself or negotiate direct with the TO/TA re any refund that might be possible? Another alternative is for you to change the name of the 2nd passenger and go with a friend who would be willing to pay you for their share of the total cost of the holiday. Your ex is due no money if you have paid all the costs to date.