Your partner may have become a little bit complacent during the honeymoon period that is the second trimester. Chances are that your discomfort was minimal and your energy levels were high.
The dawn of the third trimester often signals fatigue, back and leg pain, and a range of other discomforts. If he's not particularly sympathetic, perhaps it's time to bring him along to your next doctor's appointment so that he can hear your litany of physical complaints. Some partners are naturally sensitive, while others (ahem, mine) need to have it spelled out in black and white before they can begin to understand the physical stress your body is under.