Here's two recommendations, neither are cheap I'm afraid:

* https://thinksmartbox.com/product/grid-for-ipad/ (there's grid player as well which is free but I think requires purchasing a desktop app)

* https://www.assistiveware.com/products/proloquo2go

Depending on your country / financial situation / medical insurance you may be able to get help with these. In the UK the NHS should provide a suitable solution. Either way if at all possible speak to a qualified speech therapist and they'll be able to help with your specific situation.