Trouble in paradise...Please comment

so my SO and I have been together for three years now. It's been a generally good relationship with its ups and downs. Lately however he has had trouble getting and staying hard. This is new for him, and he is only 24. He tells me its because he's tired, hungry, or simply not in the mood. I try to be understanding, but it's killing my self esteem. I've tried going down on him, dressing sexy.. but nothing's working. Do you ladies have any advice?