Yes - if it's allowed model you can't do symbolic calculus so it won't do differentiation or integration for him. http://www.ocr.org.uk/Images/69918-faqs ... -exams.pdf
He'll have to show working ...
I've been teaching A level for 30+ years now and I use one myself and got one for my DS. It will help with checking in a couple of topics; the only possible downside is relying on it too much. If he's able enough to be doing double maths then I'm not concerned about that.
Make sure he looks at mark schemes and understands that, for example, a sketch of a graph just copied from his calculator display won't get full marks.
PM me if you want a bit more detail.