How is it spelled out in the divorce papers on visitation and transportation? If it says you split the transportation 50/50 then I would do one of two things. I would either make arrangements to get my child myself with the understanding that the father has to pay for or endure all transportation issues next time (I've had to do this with my DD's father and it has worked so far).Option two (for drastic measures) when he is not where he is supposed to be with your son when you are supposed to get him back then I would call the police and report your child as being abducted. I had to threaten this upon my DD's father one time when he was giving me grief about not being able to make the drive from MI to TN when he was supposed to give her back to me, hence why the first option works for us cause he knows I'm not playing games with him. The second option sounds harsh I know, but my DD's father has always been kind of a dead beat dad so I just decided to take a stand and put my foot down to his BS!
Just look at your divorce papers really good and see what is says you both can and can't do and go from there. And next time he is supposed to go with his dad, tell him to make sure he has the funds to get his son back home to you before he goes to visit with him. Good luck to you!!