Can you help with an ipod 4.2.1

My Son was given an ipod 4.2.1 (model MB533BT) and I want to keep in touch with him when he is at home with his Mum. Apart from email are there any wifi messenger service apps suitable for his ipod. I have tried whatsapp etc from the app store and they all say the same that this ipod needs an upgrade to 4.3 which im told cant be done - does anyone know of a good app that will work, thanks Guys