Set up iMessage-need iTunes account

I have done a search but unable to find an answer to my query so would appreciate your help.

I have a spare 3GS which I wish to give to a child relative in Thailand to keep in touch with me in the UK by iMessage.

He does not have nor can he have (because of his age) an iTunes account, he does have an email address and will have a Thai phone number.

I am happy to set the phone up on my own iTunes account but would need to resrtict him buying things.

What do you think is the best way of enabling him to use my old phone for iMessages?

All suggestions very welcome.