What was his previous diet? Just feeding cubes is not the best choice. Cubes are a very expensive way to feed hay. You pay over $400/ton ($10/50 bag) where baled hay should be $200-300 this time of year ($15-18/ 120+# bale). Cubes are also eaten faster than hay which can lead to boredom and stall vices. Cubes are also typically make out of inferior mature hay so they'll have a lower nutritional value than hay.
But that's not what you asked about. Have you opened the bran yet? Take it back if you haven't, if you have, feed it and don't sweat it. As we have a better understanding of the relationship of individual nutrients in equine nutrition, bran is no longer a feed of choice. It has a severly inverted Ca:P ratio (1:10). It's rather hard to explain this in a sentence or two but Ca and P are best when they are offered at a rate of 2X the Ca to P (2:1). Feeding 1/4# of bran is not going to upset that ration much as it's a relationship to the entire diet. Alfalfa also have an elevated Ca value that will nullify the high P. Whene fed in very high amounts, the P in bran can actually pull Ca out of bone. As a feed product, bran is pretty useless. It doesn't have any nutrient values that will enhance the diet so don't waste your $$ on it. Don't take any more advice from the tack store lady either.
I would offer at least 50% of his forage in hay. In S Ca, you have options of oat and some grass hays in addition to alfalfa. Mix it up. They typically scarf down the alfalfa than pick at the oat and grass. That's good. It gives them something in their stomach all night long as well as keeping them busy. You still need some type of vit/mineral. If he can maintain his condition on just hay, than look at ration balancers (it's a nutritent dense low feed concentrate). If he can't, than you have to explore grains and concentrates as a concentrated source of calories. You can add the joint supplement to either product. You might want to wet it down slightly to make the supplement stick to the feed and not fall to the bottom of the bucket.