Firstly thank you for raising this question with Scope.
It is very achievable for an adult to start speaking at a late age due to reasons with speech development and motor skill impairments. Proloquo2go will be very good for your son to start becoming familiar with the symbols associated with the words and phrases.
If this is the first time he is trying to communicate on his own with support from yourself, I would advise that you also use some cause and effect apps (Available on the App Store for free) which will help you understand his cognitive ability to associate symbols with words. This will also help with your son understanding the response he will get once he has made contact with the symbol on the iPad. (It is also very good for engaging your son with the iPad, so that he feels that it is fun and enjoyable instead of a challenge)
You can find some very useful tools with regards to apps on the app store at http://www.callscotland.org.uk/downloads/posters-and-leaflets/
I hope this helps and if you have any more questions, please do reply and I will be more than happy to assist you.