Maybe male so not building his nest anymore and he is only half way through

Re explain...?

My male siamese fighting fish is not building his nest any more

You need to give more information than just that like what you are trying to achieve and what you have done.

i.e. Is the male just building a nest in the tank for something to do or are you trying to breed him? Did he start building and then you decided to breed from him or have you put a female in or near him to get him to build a nest. If he stopped how many days was he building it for, what the tank is like etc etc.

The more information you give the more accurate answers you will get.

I was trying to breed him and I followed all the steps to breed them and he started and building and he half way through and he just stopped working

Just do a good water change, keep feeding him well and wait for him to decide to start again.

Sometimes males will breed without a bubble nest which is fine as long as there is something on the surface of the tank for the eggs to attach to i.e. plants.

By halfway through - what do you mean? There is no defined size for a bubble nest.