Been asked to be a personal shopper. need advice.

[family checkout]

i wish you good luck in your shopping.

[mainelycoupons]

50% of what you save him sounds fair, but figuring in time, coupons and such will take away from that. I would suggest a flat rate either per trip or per hour or whatever, plus that 50%. You may save him $100 one trip, but only $20 the next (it could happen depending on what he wants). I would suggest 50% of shelf price for your stockpile. 50% off is a fair sale for most things if you have to buy them without a coupon.
Discuss if he will compensate you extra if coupons need to be ordered or traded for special for his items, whether he will provide you with a set or two of insert coupons each week, and how you will handle scenarios which aren't cheap, or even free, but moneymakers. Overall they will balance out, but he may not be too keen on paying you 50% of 110% savings.