Most male enhancement supplements (virility ex, extenze, size genetics, enzyte, spartan blend, expanzite, etc.) contain herbs that have been shown to have strong effects on libido and circulation- and there are studies that show these effects. These herbs have been used for their sexual benefits throughout history. But, I think that he should talk to your doctor before he takes anything, in case of drug-interactions, side effects, or allergic reactions, etc.
As far as increasing the size of his penis is concerned, the herbs in male enhancement pills do not actually increase size. However, they do two other very important things: They can increase blood flow to the penis, and can help tone the pelvic floor muscles. More blood flow can lead to larger, harder erections, and toned pelvic floor muscles can make a penis stand more erect. So, they appear to increase size, but in actuality they just help you achieve your full potential.