Aside from paying all his efforts through money, I think the best reward you can give to your mentor is by seeing yourself creating improvement in your trading career. Remember that a part of you is credited from him, and your success is also his success. So whatever that made you who you are after his mentoring, it will be an honor for him seeing you have learned a lot and is now applying all your knowledge into trading.
Money is actually the main reason why mentors accept newbies for mentoring. But we should also know that if we are grateful for what they have done to us, we can always chose to give them more as long as its given through goodwill.
Our success will put them/mentors in an inner state of satisfaction where after they have given us knowledge about trading, it can be applied correctly. A teacher will definitely be happy to see his students succeed and be able to develop their potential after being taught by him. But in the case of trading, very few mentors are willing to pursue people for free because trading involves a much greater level of risk and they will definitely provide paid classes if they intend to teach people to learn trading.
There are many best gifts that can be given to mentors because they have great services in teaching us. Generally, trading mentors don't expect anything other than the payment they set when we want to learn trading, but as a form of gratitude even though we pay them there are times when we give something more which will make our relationship with them stronger.