Goodness. Poor little fella. I sure hope he gets better.
I think I would consult with the vet and find out if those legs are likely to be viable. Other than his legs he seems very active.
I second that. The splints are not helping him and there is something not right that I don't think it going to get better on it's own. It could be deficiency, could be injury, could be birth issues/neurological, but it is hard to guess. A vet call is a good idea.
Sucks that he has to be put down, but at least now you know for certain and don't waste a lot of time and effort on something that can't be fixed. Also better in the long run as you hopefully haven't become too attached to him yet. Best wishes!