Testosterone. I would definitely have his free testosterone checked it out. Best way is saliva, or serum in evening as mens testosterone levels decline as the day goes by.
See PCP. His pcp can help discern whether prob is illness related (ex. Erectile dysfunction), psychological (depression), or hormonal (low testosterone). Finding the cause is only way to treat properly. For example, viagara won't help if the problem is depression. Men are freq embarassed to bring it up, and freq it is the wife that 'forces' them to come see the dr.