Download iOS 4.3 to an original iPhone?

I gave my son my old iPhone, I want to download a phone app to it but it says I need to download 4.3 in order to install. When I plug in the phone to iTunes it says it is up to date with the most current version, so are there any phone apps that will work on a device other than an iPhone 4?

The original iphone isn't being updated anymore it was cut off since there's no point for apple as they are making more

Can't. 3.1.3 is the last firmware available for that iPhone.

IOS 3.1.3 is the best you can get for an original iPhone.
Still tons of apps available for iOS 3.1.3 or earlier. If you want to find compatible apps try,
https://itunes.apple.com/...?mt=8&uo=4
or
https://sites.google.com/site/appleclubfhs/support/advice-and-articles/finding-apps-for-older-devices