立て is the imperative form of 立つ meaning “to stand up”. So he is strongly commanding Joe to stand up as the Japanese imperative form sounds quite strong and forceful.
立つんだ is a combination of a verb dictionary form 立つ and んだ which is the broken version of のだ. A verb dictionary form followed by んだ or のだ can be used to give commands in a bit of an old-fashioned way. In English, it could be translated to “You WILL stand up!” which is not grammatically imperative but it expresses a strong command. It also sounds more encouraging and persuasive than the normal imperative form. When it’s used as a command, it sounds a bit dramatic and cheesy, so in a daily conversation you wouldn’t hear this so often, although it is often used in anime and dramas.
It can also be used to express your strong will when you want to encourage yourself.