Sounds like he is rotated too far open at contact so he may have issues driving the ball the other way. As others have said, a hitters weight is quickly shifted from the back side to the front side early in the swing and the back foot typically only has light contact with the ground or even none at all by the time the bat reaches contact. I think "staying on the back side longer" may be a little misleading.
In most cases the fix actually has much more to do with the front side than the back side. Pay attention to how the front foot is positioned when the front heal plants. If the front foot is open when the heal comes down, i.e. pointing towards the pitcher, then there is little resistance to keep the front hip from flying open and taking everything else with it. The foot should be closed off at roughly a 45 degree angle when the heal lands. This forces the front leg to lock out as the front hip rotates and restricts the front hip from opening up too far. It is normal for the front foot roll to the outer edge and then straighten out after contact as the weight begins to return to the back side.
I think Mike Trout is a good example: https://www.youtube.com/watch?v=lX_dm39fkfw