During sex, men's penises will be hard and erect as blood/sperm is pumped in to the area (the term for an erect penis is a boner). Regularly, the penis is a little floppy. The hardness helps entry into the women's vagina for reproduction. In this case, his 'thing isn't getting hard' means sex is near impossible. It also implies that he has erectile inefficiency, meaning it will be difficult to produce a child as no sperm is present.