If he is in fact truly your mentor it will go great. I mentor several young professionals all across the Country, nothing makes me happier than when they get a new better gig and move on. It is the ultimate compliment for a mentor. Nothing makes me swell with pride like getting a text from one of them thanking me for preparing them for the next step of their career. Often times I do not hear from them for extended periods of time then get a text out of the blue. Thanking me for the new promotion or change and explaining they owe me a debt they can never repay. I always explain to them the way to pay me back is to find some young aspiring individual in their circle and groom and mentor them. Then we are square!