My husband had a broken baby finger, when the doctor went to fix it, he put a splint of some sort between the fingers, when done he wrapped it then put a cast up to the elbow on him. When taking the cast off there finger is now crooked , bent and cannot be straightenedm up. My question is does he have a malpractice suit?

You start calling medical malpractice lawyers. They are good at knowing/deciding within a minute or two if they will take your case.

I broke a finger playing vollyeball. My MD friend and I went to CVS, bought a splint, and she wrapped it. It's very lumpy and doesn't bend (after 15 years). It's lumpier now even.
My elderly dad may have broken his finger in the hospital. Even they told me they 'don't do anything' to broken fingers. At best they wrap it to the next finger.

Why did he have a cast on his arm?

Maybe. If the finger was not reset properly there could have been malpractice. You will need to subpoena the x-rays and have them reviewed by another physician. It is possible the fracture was bad enough that healing straight was a chance. Give a full arm cast, I suspect the doctor was concerned about healing and that's why he went to extremes to immobilize it.

My guess here is that the doctor acted appropriately, but without seeing the x-rays I couldn't be sure.

It is possible, to be honest, it actually sounds like they over treated. Not sure how long ago, but on a finger, taping two fingers together with a plastic splint is about it, no wrap up to the elbow, just two fingers wrapped. Depending on where the break is, sometimes usage can be restricted.

But you take a copy of all of the medical records and have one or two other doctors review it. Since you will need another medical professional who will testify in court that it was not treated properly