He’s just not all that interested.
In the moments when he messages you and is talking about hooking up, he may actually be interested in doing just that.
However, he constantly bails on you because by the time you’re supposed to meet him for the hookup, he may have already gotten another girl to agree to hook up with him.
There’s a good chance that you are not the only one that he’s constantly messaging about hooking up.
Hence, you are most likely not at the top of his priority list when it comes to hooking up and hanging out.
He may only look at you as someone that could be a possible replacement if some other girl that he’s more interested in hooking up with doesn’t follow through.