Yes, Mandarin is pretty difficult as we hear it often, but I think we should not judge our children's grasping power to what we hear. Children have high grasping ability and aren't there kids who learn Madarin! But now that you have already making him learn Spanish, I think you should not repent your decision. Spanish is widely spoken language across the world and you can work him have perfection over the language. But I suggest do not underestimate or go on what people say about any craft, language or skill. Let your child try it and see how he is excelling in it and then decide what to do in tandem with your child.