The only problem with Gridplayer Ouryve, is that unless you have The Grid (which costs money!) on your main computer, you can't edit the program, and personalise it. It's a good programme, but even the best programs still need personalising. Before you commit to it, I would decide whether you would be able long term to purchase The Grid as well. The other slightly annoying thing is that because you can only edit within the Grid, you can't quickly add things, for instance, when you're out and about.
Also, on the iPad, you are limited to certain symbol sets - Widget (good!), Makaton (not so good!), and symbolpics I think. You can't use PCS, which is very widely used in schools, due to licensing issues.
That might not matter at all. I'm sorry I can't remember how old your son is, but if he is in school (or about to be), where a particular symbol set is used, then I would go with that. Children learn symbols (when they're working well for them) as a language - so switching from PCS to widget (for example), would be like switching from Italian to Spanish.
I'm personally not a fan of the iPad as a platform for communication programmes. There are millions to choose from, but I've yet to see one that I really like and would recommend. I would at present always suggest an android system, as - for instance - you can run The Grid directly onto that. And then you have masses more (quality) choice of programmes.
Always good to get some specialist advice though if you can - to make an AAC system work it needs to be completely personalise and tailored to your son and your situation, so getting it right is key. Do you have a specialist SLT service you can ask for referral too? If he's getting on well with PECS it sounds like you're right to seize the moment and try to move him on!