My DS also uses P2G. In the beginning it was great, then DS seemed self conscious of it. He would refuse to use it in school but would be happy using it at home/ small group classes. I also suspect, as his school weren’t using it with him, that also increased his issue with it.
Right now, we follow his lead. He doesn’t use it at school but we use it together at home.
It could be the programme your DS isn’t keen on. I would get your DS assessed by one of the AAC hubs, which are all around in the UK. You would be seen by an AAC specialist, who would assess your son on different types of AAC programmes and grid sizes etc.
Can I also ask... is your grid size limited ? If it is, your DS May be frustrated by a small grid size. On the Proloquo2go, DS has a grid size of 8X11 or something. They recommend you increase your grid size up by 7X7 or 7X11...
The OCD type behaviour will go. It’s almost like playing with a new toy, you explore with it a bit, over and over again and once you know how to play with it, you use it functionally. Like the P2G, the more you model on the device, modelling different times in the day, different things, the more your DS will learn how to use and the OCD behaviours will eventually go.
I recommend you join the coreword classroom, by Assitive ware, Google this. It’s free and lists a number of activities you can use alongside your AAC.
It’s great that your DS is using his device to request for things and I’m sure your already doing this, but also model other functions that are a bit more difficult. For example: I DONT LIKE THAT”, “I’M BORED”, “THESE PANCAKES ARE YUCKY”.