Welcome to the forum! Please note I do not any personal experience with this, but I would give it a shot now. He will still be connecting the visual input of the words and the pictures/videos together and there is a good likelihood he will enjoy the visual aspects of the program. If he is beginning to recognize words, he may even be able to match them to items if you print off the cards (like a matching game). That aspect doesn't really have anything to do with hearing, you are isolating the skill of visual recognition.
Keep it fun and since you are aware that his device is going to be fine tuned and he will be hearing better, then the sounds will catch up time anyway. I am guessing he won't get as much out of the phonics or the game portion of the program if he does not have hearing at this point, although I could be wrong if he is reading lips and can see you pronounce the words. Follow his lead and present it in a very happy, super positive way and see how he responds. If he has any hearing at all, I would definitely encourage turning up the volume and using it! Let us know what you decide.
Edited to add: I just reread that he is going to speech therapy. Then yes, this program will most likely be very helpful to him them, most parents notice an increase is speech and vocabulary while using it. I like this website too:
http://mommyspeechtherapy.com/?page_id=55
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