Maths would look better, but Psychology is not a soft subject. It has one of the biggest workloads of all the A Levels, it can be very hard in parts, there's a lot to learn and understand, and at no point can you sail through. However, Medicine is a very competitive course to get into, and the vast majority of candidates will have 3 Sciences, 2 Sciences and Maths, or even 3 Sciences and Maths. If he doesn't think that he'll be able to get an A/A* in Maths then he shouldn't carry on with it, but if he can, he should.