Hi Alicia, and welcome back to Just Answer.
I'm so sorry to hear this has happened to poor Spirit.
For the moment, you did what you had to do and if you bring him to his regular vet in the morning, you will be able to have an x-ray done at a lower cost and see what's actually happening with the leg. It could be broken, it could have a hairline fracture, a joint could be dislocated. There are many possibilities, but an x-ray will certainly show more and if the splint is not positioned correctly, after a diagnosis is made of the injury, your vet will re-do it correctly. It's important that the splint (or even if a cast is needed for a break/fracture) is positioned right, so the bone heals properly and doesn't cause future problems. You will not know if surgery is needed until your vet makes a diagnosis after the x-ray.
If Spirit is eating and walking gingerly, this is very good; however, I would try to keep him off his feet and confined, until you can get him to your vet in the morning. He also should be prevented from jumping up onto or down from, anything. If you have a large dog crate or something similar, you can put him in there, with a few layers of towels or blankets on the bottom and his essentials: food, water, litterbox, and allow him to rest. I'm assuming the ER vet gave him some pain medication, so that should help him feel more comfortable.
I hope all will be well with Spirit, once he is evaluated and treated by his regular vet.
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